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Katsuya Fukuda

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Proceedings Papers

*Proc. ASME*. IMECE2001, Heat Transfer: Volume 2 — Heat Transfer in Multiphase Systems, 127-149, November 11–16, 2001

Paper No: IMECE2001/HTD-24163

Abstract

The mechanisms and the correlations for the subcooled water flow boiling CHF, q ct,sub , for outlet subcooling at fixed flow velocities with the pressure as a parameter measured were investigated using the existing database measured for the test sections such as a vertical metal strip in the channel with a wide cross section, and the vertical tubes having the inside diameters ranging from 0.4 to 12 mm and the length-to-diameter ratio ranging from 2.4 and 96.6 with water flowing upward based on the unified subcooled pool and flow boiling CHF correlations previously presented: the unified CHF correlations based on the two different mechanisms due to the hydrodynamic instability (HI) and the heterogeneous spontaneous nucleation (HSN). The q ct,sub versus outlet subcooling, ΔT sub,out , for the test tubes at a flow velocity with the pressure were clearly divided into four regions for the ΔT sub,out in general: namely, first, the q ct,sub decrease down to minimum one for low ΔT sub,out (1st Region), secondly, the q ct,sub increase up to maximum one for middle ΔT sub,out (2nd Region), thirdly, the q ct,sub considerably decrease down to minimum one for transition ΔT sub,out (3rd Region), and finally, the q ct,sub again increase monotonously for high ΔT sub,out (4th Region) with an increase in ΔT sub,out . The trend of the q ct,sub versus the ΔT sub,out , in the third region for the transition ΔT sub,out became significant with a decrease in tube inside diameter though the trend of the q ct,sub for the tubes having large inside diameters of 9 and 12 mm became insignificant. It was clarified that the q ct,sub data for middle ΔT sub,out (2nd Region) and those for high ΔT sub,out (4th Region) for the test sections with various shapes of the metal strip and the tubes having the inside diameters ranging from 0.4 to 12 mm and length-to-diameter ratio ranging from 2.4 to 96.6 were well expressed by the unified subcooled water flow boiling CHF correlations derived based on the existing database.

Journal Articles

Article Type: Research Papers

*. December 2020, 12(6): 061002.*

*J. Thermal Sci. Eng. Appl*Paper No: TSEA-19-1466

Published Online: June 16, 2020

Abstract

The boiling heat transfer for subcooled water flowing in a small-diameter tube was investigated experimentally and numerically. In the experiment, a platinum tube was used as an experimental tube (d = 1.0–2.0 mm) to conduct joule heating by direct current. The heat generation rate of the tube was controlled with an exponential function. The numerical simulation of boiling heat transfer for subcooled water flowing in the small-diameter tube was conducted using the commercial computational fluid dynamics (CFD) code, phoenics ver. 2013. The small-diameter tube was modeled in the simulation. As the boundary condition, the measured heat flux was given at the inner wall. The inlet temperature ranged from 302 to 312 K. The flow velocities of d = 1.0 mm and d = 2.0 mm were 9.29 m/s and 2.34 m/s, respectively. The three-dimensional analysis was carried out from non-boiling to the critical heat flux (CHF). Governing equations were discretized using the finite volume method in the phoenics . The semi-implicit method for pressure linked equation (SIMPLE) method was applied in the numerical simulation. For modeling boiling phenomena in the tube, the Eulerian–Eulerian two-fluid model was adopted using the interphase slip algorithm of phoenics . The surface temperature difference increased as the heat flux increased in the experiment. The numerical simulation predicted the experimental data well. When the heat flux of the experiment reached the CHF point, the predicted value of the heat transfer coefficient was approximately 3.5% lower than that of the experiment.

Journal Articles

Article Type: Research Papers

*. December 2020, 12(6): 061004.*

*J. Thermal Sci. Eng. Appl*Paper No: TSEA-19-1487

Published Online: June 16, 2020

Abstract

This paper presents an experimental investigation of the forced convective heat transfer of FC-72 in vertical tubes at various velocities, inlet temperatures, and tube sizes. Exponentially escalating heat inputs were supplied to the small tubes with inner diameters of 1, 1.8, and 2.8 mm and effective heated lengths between 30.1 and 50.2 mm. The exponential periods of heat input range from 6.4 to 15.5 s. The experimental data suggest that the convective heat transfer coefficients increase with an increase in flow velocity and µ/µ w (refers to the viscosity evaluated at the bulk liquid temperature over the liquid viscosity estimated at the tube inner surface temperature). When tube diameter and the ratio of effective heated length to inner diameter decrease, the convective heat transfer coefficients increase as well. The experimental data were nondimensionalized to explore the effect of Reynolds number (Re) on forced convection heat transfer coefficient. It was found that the Nusselt numbers (Nu) are influenced by the Re for d = 2.8 mm in the same pattern as the conventional correlations. However, the dependences of Nu on Re for d = 1 and 1.8 mm show different trends. It means that the conventional heat transfer correlations are inadequate to predict the forced convective heat transfer in minichannels. The experimental data for tubes with diameters of 1, 1.8, and 2.8 mm were well correlated separately. And, the data agree with the proposed correlations within ±15%.

Proceedings Papers

*Proc. ASME*. HT2019, ASME 2019 Heat Transfer Summer Conference, V001T10A013, July 14–17, 2019

Paper No: HT2019-3699

Abstract

In this research, the transient heat transfer due to exponentially increasing heat input was experimentally measured for upward water flowing in a vertical small tube. The heat generation rate was increased exponentially with a function of Qoexp ( t /τ), where, Qo is an initial heat generation rate, t represents time and τ is e-folding time. The heat generation rate was controlled by high speed computer system. The test tube was heated with exponentially increasing heat input by direct current. The average temperature of test tube was measured by resistance thermometry using a double bridge circuit. The experimental apparatus consists of a test section, a cooler, a heater, a pump, a tank and a pressurizer. The working fluid was distilled and deionized water. The inlet fluid temperature of test tube was controlled by the cooler and the heater. The system pressure was up to 800 kPa. The test tube was 0.7 mm in inner diameter and 12.0 mm in heated length respectively. The ratio of heated length to inner diameter was 17.1. The test tube was electrically isolated from experimental loop by Bakelite plates. The experimental data were compared with previous correlations of nucleate boiling. It was obtained that the experimented data agree well with full-developed flow boiling correlation by Rohsenow. Moreover, the transient critical heat flux (CHF) and nucleate boiling with onset of nucleate boiling (ONB) values increased with the increase in flow velocity. The transient CHFs and ONBs increased with a decrease in e-folding time at τ < 1 s, and they approached steady-state value at τ > 1 s. It was understood that the heat transfer is in steady-state at τ > 1 s, and it is in transient state at τ < 1 s.

Proceedings Papers

*Proc. ASME*. MNHMT2019, ASME 2019 6th International Conference on Micro/Nanoscale Heat and Mass Transfer, V001T04A007, July 8–10, 2019

Paper No: MNHMT2019-4163

Abstract

Numerical simulation of boiling heat transfer for subcooled water flowing in a small-diameter tube was conducted using the commercial computational fluid dynamics (CFD) code, PHOENICS ver. 2013. A small-diameter tube (d = 1.0–2.0 mm) was modeled in the simulation. A uniform heat flux with an exponential function was given at the inner tube wall as the boundary conditions. The inner wall boundary condition was set to a non-slip. The inlet temperature ranged from 302 to 312 K. The flow velocities of d = 1.0 mm and d = 2.0 mm are 9.29 m/s and 2.34 m/s, respectively. The transient analysis was carried out from the non-boiling region since the heat flux increased with time in the author’s experiments. The governing equations including the energy equation were discretized using the finite volume method in the PHOENICS code. The SIMPLE method was applied for the numerical simulation. For modeling boiling phenomena in the tube, the Eulerian-Eulerian two-fluid model was adopted using the interphase slip algorithm of PHOENICS code. In the experiment, a platinum tube was used as the experimental tube (d = 1.0–2.0 mm) to conduct joule heating by direct current. The distilled and deionized water was pressured by the pressurizer. The heat generation rate of the tube was controlled with the exponential function to obtain the transient heat transfer characteristics from the non-boiling region. The surface superheat increased as the heat flux increased in the experiment. The numerical simulation predicted the experimental data well. When the heat flux of the experiment was reached to the CHF point, the predicted value of heat transfer coefficient was approximately 3.5 % lower than that of the experiment.

Proceedings Papers

*Proc. ASME*. MNHMT2019, ASME 2019 6th International Conference on Micro/Nanoscale Heat and Mass Transfer, V001T11A011, July 8–10, 2019

Paper No: MNHMT2019-4086

Abstract

In this paper, the forced convective heat transfer of FC-72 was experimentally investigated for various of parameters like velocity, inlet temperature, tube size, and exponential period of heat generation rate. Circular tubes with different inner diameters (1, 1.8 and 2.8 mm) and heated lengths (30–50 mm) were used in this study. The experiment data suggest that the single-phase heat transfer coefficient increases with increasing flow velocity as well as decreasing tube diameter and ratio of heated length to inner diameter. The experiment data were nondimensionalized to study the effect of Reynolds number (Re d ) on forced convection heat transfer. The results indicate that the relation between Nusselt numbers (Nu d ) and Re d for d = 2.8 mm show the same trend as the conventional correlations. However, the Nu d for d = 1 and 1. 8 mm depend on Re d in a different manner. The conventional heat transfer correlations are not adequate for prediction of forced convective heat transfer in mini channels. The heat transfer correlations for FC-72 in vertical small tubes with diameters of 1, 1.8 and 2.8 mm were developed separately based on the experiment data. The differences between experimental and predicted Nu d are within ±15%.

Topics:
Convection

Journal Articles

Article Type: Research-Article

*. April 2019, 5(2): 021002.*

*ASME J of Nuclear Rad Sci*Paper No: NERS-17-1088

Published Online: March 15, 2019

Abstract

Laminar natural convection heat transfer from vertical 7 × 7 rod bundle in liquid sodium was numerically analyzed to optimize the thermal–hydraulic design for the bundle geometry with equilateral square array (ESA). The unsteady laminar three-dimensional basic equations for natural convection heat transfer caused by a step heat flux were numerically solved until the solution reaches a steady-state. The code of the parabolic hyperbolic or elliptic numerical integration code series (PHOENICS) was used for the calculation considering the temperature dependence of thermophysical properties concerned. The 7 × 7 heated rods for diameter (D = 0.0076 m), length (L = 0.2 m) and L/D (=26.32) were used in this work. The surface heat fluxes for each cylinder, which was uniformly heated along the length, were equally given for a modified Rayleigh number, (Ra f,L ) ij and (Ra f,L ) Nx×Ny,S/D , ranging from 3.08 × 10 4 to 4.28 × 10 7 (q = 1 × 10 4 ∼7 × 10 6 W/m 2 ) in liquid temperature (T L = 673.15 K). The values of ratio of the diagonal center-line distance between rods for bundle geometry to the rod diameter (S/D) for vertical 7 × 7 rod bundle were ranged from 1.8 to 6 on the bundle geometry with ESA. The spatial distribution of average Nusselt numbers for a vertical single cylinder of a rod bundle, (Nu av ) ij , and average Nusselt numbers for a vertical rod bundle, (Nu av ,B ) Nx×Ny,S/D , were clarified. The average values of Nusselt number, (Nu av ) ij and (Nu av ,B ) Nx×Ny,S/D , for the bundle geometry with various values of S/D were calculated to examine the effect of array size, bundle geometry, S/D, (Ra f,L ) ij and (Ra f,L ) Nx×Ny,S/D on heat transfer. The bundle geometry for the higher (Nu av ,B ) Nx×Ny,S/D value under the condition of S/D = constant was examined. The general correlations for natural convection heat transfer from a vertical N x ×N y rod bundle with the ESA and equilateral triangle array (ETA), including the effects of array size, (Ra f,L ) Nx×Ny,S/D and S/D were derived. The correlations for vertical N x ×N y rod bundles can describe the theoretical values of (Nu av ,B ) Nx×Ny,S/D for each bundle geometry in the wide analytical range of S/D (=1.8–6) and the modified Rayleigh number ( (Ra f,L ) Nx×Ny,S/D = 3.08 × 10 4 to 4.28 × 10 7 ) within −9.49 to 10.6% differences.

Proceedings Papers

*Proc. ASME*. ICONE26, Volume 9: Student Paper Competition, V009T16A023, July 22–26, 2018

Paper No: ICONE26-81391

Abstract

Knowledge of the heat transfer phenomenon under flow decay transient condition is important for the safety assessment of a very high temperature reactor (VHTR) during a loss of coolant accident. In this study, transient heat transfer from a horizontal cylinder to helium gas under exponentially decreasing flow rate condition was experimentally investigated. The experiment was conducted by using a forced convection heat transfer experimental apparatus. A flow control value with its control system was used to realize a flow decay condition. Helium gas was used as a coolant, and a platinum cylinder with a diameter of 1 mm was used as the test heater. A uniform heat generation rate was added to the cylinder by a power source. The cylinder temperature was maintained at an initial value under a definite initial flow rate of the helium gas. Subsequently, the flow rate of the helium gas began to exponentially decrease with different time constants ranging from 3 s to 15 s. The initial flow velocity ranged from 7 m/s to 10 m/s. The surface temperature, heat flux, and heat transfer coefficient were measured during the flow decay transient process under a wide range of experimental conditions such as heat generation rates and flow decay time constants. The results indicated that the temperature of the test heater exhibits a rapid increase during this process, and the increasing rate of the temperature is higher for a lower time constant. An increase in the heat generation rate leads to a higher increase in the surface temperature. Therefore, the heat generation rates of the fuel rods are high when a VHTR operates at high power, and it is more challenging to implement passive safety design to ensure the temperature limitation of the fuel rods during a loss-of-coolant accident. Moreover, the heat transfer coefficient relative to time during the flow rate decreasing process was also obtained. The transient heat transfer process during exponentially decreasing flow rate condition was examined based on the experimental data.

Proceedings Papers

*Proc. ASME*. HT2017, Volume 2: Heat Transfer Equipment; Heat Transfer in Multiphase Systems; Heat Transfer Under Extreme Conditions; Nanoscale Transport Phenomena; Theory and Fundamental Research in Heat Transfer; Thermophysical Properties; Transport Phenomena in Materials Processing and Manufacturing, V002T14A009, July 9–12, 2017

Paper No: HT2017-5043

Abstract

Critical heat flux (CHF) of convective boiling in a mini-tube due to power transient was measured. A platinum tube with an inner diameter of 1.0 mm was heated exponentially by a direct current power supply as Joule heating. The heated length of the platinum tube was 40.9 mm. The platinum tube was mounted vertically in the water-loop apparatus which consisted of a circulating pump, a pre-heater, a flow mater, a pressurizer, a cooler and a test section. The deionized water was pressurized by the pressurizer up to approximately 800 kPa to measure CHFs at the high subcooling. The upward flow velocity in the platinum tube was ranged from 5 to 11 m/s. The inlet subcooling was ranged from 92 to 117 K. The heat generation rate was controlled with exponential functions. The e-folding time of the heat generation rate was ranged from 30 ms to 18 s. As an experimental result, it was found that the CHFs increased with increasing the flow velocity and the inlet subcooling. The CHF also increased with decreasing the e-folding time of the heat generation rate. Since the heat generation rate of the platinum tube increased rapidly under the power transient condition, it was considered that the heat flux of the platinum tube increased until the vapor blanket covered the heated surface of the platinum tube.

Proceedings Papers

*Proc. ASME*. ICONE25, Volume 6: Thermal-Hydraulics, V006T08A010, July 2–6, 2017

Paper No: ICONE25-66180

Abstract

Natural convection heat transfer from vertical 7×7 rod bundle in liquid sodium was numerically analyzed to optimize the thermal-hydraulic design for the bundle geometry with equilateral square array, ESA. The unsteady laminar three dimensional basic equations for natural convection heat transfer caused by a step heat flux were numerically solved until the solution reaches a steady-state. The PHOENICS code was used for the calculation considering the temperature dependence of thermo-physical properties concerned. The 7×7 test rods for diameter ( D = 7.6 mm), heated length ( L = 200 mm) and L/d (= 26.32) were used in this work. The surface heat fluxes for each cylinder were equally given for a modified Rayleigh number, ( R f,L ) ij and (R f,L ) Nx×Ny,S/D , ranging from 3.08×10 4 to 4.28×10 7 ( q = 1×10 4 ∼7×10 6 W/m 2 ) in liquid temperature ( T L = 673.15 K). The values of S/D , which are ratios of the diameter of flow channel for bundle geometry to the rod diameter, for vertical 7×7 rod bundle were ranged from 1.8 to 6 on the bundle geometry with equilateral square array. The spatial distribution of average Nusselt numbers for a vertical single cylinder of a rod bundle, (Nu av ) ij , and average Nusselt numbers for a vertical rod bundle, (Nu av,B ) Nx×Ny,S/D , were clarified. The average value of Nusselt number, (Nu av ) ij and (Nu av,B ) Nx×Ny,S/D , for the bundle geometry with various values of S/D were calculated to examine the effect of array size, bundle geometry, S/D , (R f,L ) ij and (R f,L ) Nx×Ny,S/D on heat transfer. The bundle geometry for the higher (Nu av,B ) Nx×Ny,S/D value under the condition of S/D = constant was examined. The general correlations for natural convection heat transfer from a vertical N x ×N y rod bundle with the equilateral square and triangle arrays including the effects of array size, (R f,L ) Nx×Ny,S/D and S/D were derived. The correlations for vertical N x ×N y rod bundles can describe the theoretical values of (Nu av,B ) Nx×Ny,S/D for each bundle geometry in the wide analytical range of S/D (= 1.8 to 6) and the modified Rayleigh number ( (R f,L ) Nx×Ny,S/D = 3.08×10 4 to 4.28×10 7 ) within −9.49 to 10.6 % differences.

Proceedings Papers

*Proc. ASME*. ICONE25, Volume 6: Thermal-Hydraulics, V006T08A032, July 2–6, 2017

Paper No: ICONE25-66469

Abstract

Knowledge of the heat transfer phenomenon during flow decay transient condition is important for the safety assessment of very high temperature reactor (VHTR) during the loss of coolant accident. In this study, transient heat transfer from a horizontal cylinder to helium gas under exponentially decreasing flow rate condition was experimentally studied. The experiment was performed by using a forced convection heat transfer test loop. A flow control value with its control system was used to realize the flow decay condition. Helium gas was used as coolant and platinum cylinder with 1 mm in diameter was used as the test heater. A uniform heat generation rate was added to the cylinder by a power source. The cylinder temperature was maintained at an initial value under a definite initial flow rate of the helium gas. Then, the mass flow rate of the helium gas starts to decrease exponentially with different time constants ranged from 4.3 s to 15.4 s. The initial flow velocity ranged from 10 m/s to 4 m/s. The surface temperature, heat flux, and heat transfer coefficient were measured during the flow decay transient process under wide experimental conditions such as initial flow rate, flow decay time constant. It was found that the temperature of the test heater shows rapid increase during this process, the increasing rate of the temperature is higher for a shorter time constant. The heat transfer coefficient versus time during the flow rate decreasing process was also obtained. The transient heat transfer process during exponentially decreasing flow rate condition was clarified based on the experimental data.

Journal Articles

Journal:
Journal of Heat Transfer

Article Type: Research-Article

*. March 2017, 139(3): 032502.*

*J. Heat Transfer*Paper No: HT-16-1447

Published Online: January 4, 2017

Abstract

Natural convection heat transfer from vertical 5 × 5 rod bundles in liquid sodium was numerically analyzed for two types of the bundle geometry (equilateral square array (ESA) and equilateral triangle array (ETA)). The unsteady laminar three-dimensional basic equations for natural convection heat transfer caused by a step heat flux were numerically solved until the solution reaches a steady-state. The phoenics code was used for the calculation considering the temperature dependence of thermophysical properties concerned. The 5 × 5 test rods for diameter (D = 7.6 mm), heated length (L = 200 mm), and L/d (=26.32) were used in this work. The surface heat fluxes for each cylinder were equally given for a modified Rayleigh number, (R f , L ) ij and (R f , L ) 5 × 5, S / D , ranging from 3.08 × 10 4 to 4.19 × 10 7 (q = 1 × 10 4 –7 × 10 6 W/m 2 ) in liquid temperature (T L = 673.15 K). The values of S/D, which are ratios of the diameter of flow channel for bundle geometry to the rod diameter, for vertical 5 × 5 rod bundles were ranged from 1.8 to 6 on each bundle geometry. The spatial distribution of local and average Nusselt numbers, (Nu av ) ij and (Nu av, B ) 5 × 5, S / D , on vertical rods of a bundle was clarified. The average value of Nusselt numbers, (Nu av ) ij and (Nu av, B ) 5 × 5, S / D , for the two types of the bundle geometry with various values of S/D were calculated to examine the effect of the bundle geometry, S/D, (R f , L ) ij , and (R f , L ) 5 × 5, S / D on heat transfer. The bundle geometry for the higher (Nu av, B ) 5 × 5, S / D value under the condition of S/D = constant was examined. The correlations for (Nu av, B ) 5 × 5, S / D for two types of bundle geometry above mentioned including the effects of (R f , L ) 5 × 5, S / D and S/D were developed. The correlations can describe the theoretical values of (Nu av, B ) 5 × 5, S / D for the two types of the bundle geometry at S/D ranging from 1.8 to 6 within −12.64% to 7.73% difference.

Proceedings Papers

*Proc. ASME*. ICONE24, Volume 3: Thermal-Hydraulics, V003T09A062, June 26–30, 2016

Paper No: ICONE24-60783

Abstract

The Very High Temperature Reactor (VHTR) is a new reactor that uses helium gas as primary coolant for conventional graphite matrix, coated fuels. It enables the achievements of high thermal efficiency and can supply heat with a high temperature of about 900–1000 °C. During the loss of coolant process, the fuel hot spot temperature should not get over a criteria value due to the temperature limitation of the fuel assembly. Traditionally, the VHTRs are designed to deal with loss of forced circulation conditions by using a passive mode decay heat removal system for the cavity cooling. However, even passive systems may experience some failure due to multiple undesired conditions even though the possibility is extremely low. Therefore, the VHTRs are now expected to be designed as naturally safe reactors with inherent safety features. Which means the decay heat removal is fully dependent on natural convection and radiation. To accomplish the tough task, a clear understanding of the heat transfer process during flow decay transient condition is quite necessary. This study was conducted to investigate the transient heat transfer process between the solid surface and coolant (helium gas) in VHTR under flow decay conditions. Forced convection transient heat transfer for a horizontal cylinder under flow decay transient condition was experimentally studied. The experiment was conducted by using the helium gas as coolant. A uniform heat generation rate was added to the heater. With a certain flow rate of the helium gas, the heater temperature was maintained at a designed value. Then, the flow rate of the helium gas starts to decrease according to designed linear functions with different decreasing speed. Platinum cylinder with 1 mm in diameter was used as the test heater. The heat transfer coefficient and surface temperature were measured during the flow decay transient process under wide experimental conditions such as initial flow rate, flow decay time. It was found that the temperature of the test heater increases in curve shape with different gradients during this process, with a shorter flow decay time the increasing rate of heater surface temperature would be higher. The heat transfer coefficient versus time during the flow rate decreasing process was also obtained.

Proceedings Papers

*Proc. ASME*. ICONE24, Volume 3: Thermal-Hydraulics, V003T09A007, June 26–30, 2016

Paper No: ICONE24-60180

Abstract

Natural convection heat transfer from vertical 5×5 rod bundles in liquid sodium was numerically analyzed for two types of the bundle geometry (equilateral square and triangle arrays, ESA and ETA). The unsteady laminar three dimensional basic equations for natural convection heat transfer caused by a step heat flux were numerically solved until the solution reaches a steady-state. The PHOENICS code was used for the calculation considering the temperature dependence of thermophysical properties concerned. The 5×5 test rods for diameter ( D = 7.6 mm), heated length ( L = 200 mm) and L/d (= 26.32) were used in this work. The surface heat fluxes for each cylinder were equally given for a modified Rayleigh number, (R f,L ) ij and (R f,L ) 5×5,S/D , ranging from 3.08 × 10 4 to 4.19 × 10 7 ( q = 1 × 10 4 ∼7 × 10 6 W/m 2 ) in liquid temperature ( T L = 673.15 K). The values of S/D , which are ratios of the diameter of flow channel for bundle geometry to the rod diameter, for vertical 5×5 rod bundles were ranged from 1.8 to 6 on each bundle geometry. The spatial distribution of local and average Nusselt numbers, (Nu av ) ij and (Nu av,B ) 5×5,S/D , on vertical rods of a bundle was clarified. The average value of Nusselt number, (Nu av ) ij and (Nu av,B ) 5×5,S/D , for two types of the bundle geometry with various values of S/D were calculated to examine the effect of the bundle geometry, S/D , (R f,L ) ij and (R f,L ) 5×5,S/D on heat transfer. The bundle geometry for the higher (Nu av,B ) 5×5,S/D value under the condition of S/D = constant was examined. The correlations for (Nu av,B ) 5×5,S/D for two types of bundle geometry above mentioned including the effects of (R f,L ) 5×5,S/D and S/D were developed. The correlations can describe the theoretical values of (Nu av,B ) 5×5,S/D for two types of the bundle geometry for S/D ranging from 1.8 to 6 within −11.77 to 13.34 % difference.

Proceedings Papers

*Proc. ASME*. ICONE22, Volume 2A: Thermal Hydraulics, V02AT09A003, July 7–11, 2014

Paper No: ICONE22-30027

Abstract

The flow transient critical heat fluxes (FT-CHFs, q cr,sub ) in a SUS304-circular tube caused by a rapid decrease in velocity from non-boiling regime are systematically measured for initial flow velocities ( u 0 =7.057 to 13.635 m/s for conditions of u 0 =6.9, 9.9 and 13.3 m/s), initial heat fluxes ( q 0 =15.59 to 17.34 MW/m 2 ), inlet liquid temperatures ( T in =290.12 to 308.51 K), outlet pressures ( P out =698.38 to 1288.97 kPa) and decelerations caused by a rapid decrease in velocity ( u(t) = u 0 + αt , α =−7.357 to −0.326 m/s 2 ) by the experimental water loop comprised of a multistage canned-type circulation pump controlled by an inverter. The SUS304-circular tubes of inner diameter ( d =6 mm), heated length ( L =59.5 to 59.7 mm), effective length ( L eff =48.7 to 50.2 mm), L/d (=9.92 to 9.95), L eff /d (=8.12 to 8.37) and wall thickness ( δ =0.5 mm) with average surface roughness ( Ra =3.89 μm) are used in this work. The flow transient CHFs for SUS304-circular tube are compared with authors’ steady-state CHF data for the empty VERTICAL and HORIZONTAL SUS304-circular tubes and the values calculated by authors’ steady-state CHF correlations against outlet and inlet subcoolings for the empty circular tube. The influences of initial flow velocity ( u 0 ), initial heat flux ( q 0 ) and deceleration caused by a rapid decrease in velocity ( α ) on the flow transient CHF are investigated into details and the widely and precisely predictable correlations of CHF and flow velocity at the flow transient CHF for the circular tube is given based on the experimental data. The correlations can describe the flow velocity and the CHFs at the flow transient CHFs for SUS304-circular tube obtained in this work within ±20 % difference.

Proceedings Papers

*Proc. ASME*. ICONE22, Volume 5: Innovative Nuclear Power Plant Design and New Technology Application; Student Paper Competition, V005T17A022, July 7–11, 2014

Paper No: ICONE22-30158

Abstract

Transient forced convection heat transfer due to exponentially increasing heat input to a heater is important as a database for safety assessment of the transient heat transfer process in a Very High Temperature Reactor (VHTR). The knowledge of heat transfer enhancement using a heater with twisted configuration is also important for the high performance design of intermediate heater exchanger (IHX) in VHTR system. In this study, forced convection transient heat transfer for helium gas at various periods of exponential increase of heat input to a short thin twisted plate with various helix angles was experimentally studied. A forced convection heat transfer experimental apparatus was used to measure the experimental data. The test heater was mounted horizontally along the center part of a circular test channel. Twisted plates were made of thin platinum plate with a thickness of 0.1 mm and width of 2 mm and 4 mm. The heat generation rates of the heater were controlled and measured by a heat input control system. The heat generation rate, Q̇ , was raised with exponential function, Q̇ = Q 0 exp( t /τ). Where, t is time, and τ is period of heat generation rate. The mean temperature of the test heater was measured by resistance thermometry. The heat flux was obtained by the energy conservation equation. The test heater surface temperature was calculated from heat conduction equation of the heater. The transient heat transfer experimental data were measured for the periods ranged from 80 ms to 17 s and at a gas temperature of 303 K under 500 kPa. The flow velocities ranged from 4 m/s to 10 m/s. In the experiments, the twisted plates with different width were tested. The surface temperature and heat flux are increasing exponentially with the time. It was clarified that the heat transfer coefficient approaches the quasi-steady-state one for the period longer than about 1 s, and it becomes higher for the period shorter than about 1 s. The heat transfer coefficients for total length of the twisted plate were compared with the values of flat plate which has the same width and thickness with the twisted one. The local mean heat transfer coefficients have been tested as well. The heat transfer coefficients of twisted plate are about 10% for 2 mm-width one and15% for 4 mm-width one higher than those of flat plate with same width at the quasi-steady state. And also, the heat transfer coefficients for the first half pitch are 24% higher than that for the total length of the same twisted plate. Therefore, an enhancement in the heat transfer coefficient for the twisted plate was clarified.

Journal Articles

Article Type: Research-Article

*. September 2014, 6(3): 031010.*

*J. Thermal Sci. Eng. Appl*Paper No: TSEA-13-1176

Published Online: March 17, 2014

Abstract

The transient critical heat fluxes (transient CHFs) in SUS304-circular tubes with various twisted-tape inserts are systematically measured for mass velocities (G = 3988–13,620 kg/m2s), inlet liquid temperatures (Tin = 287.55–313.14 K), outlet pressures (Pout = 805.11–870.23 kPa) and exponentially increasing heat inputs (Q = Q0 exp(t/τ), exponential periods, τ, of 28.39 ms to 8.43 s) by the experimental water loop comprised of a multistage canned-type circulation pump controlled by an inverter. The SUS304-circular tube of inner diameter (d = 6 mm), heated length (L = 59.4 mm), effective length (Leff = 49.4 mm), L/d (=9.9), Leff/d (=8.23), and wall thickness (δ = 0.5 mm) with average surface roughness (Ra = 3.89 μm) is used in this work. The SUS304 twisted-tapes with twist ratios, y [H/d = (pitch of 180 deg rotation)/d], of 2.40 and 4.45 are used. The transient critical heat fluxes for SUS304-circular tubes with the twisted-tapes of y = 2.40 and 4.45 are compared with authors' transient CHF data for the empty SUS304-circular tube and a SUS304-circular tube with the twisted-tape of y = 3.37, and the values calculated by authors' transient CHF correlations for the empty circular tube and the circular tube with twisted-tape insert. The influences of heating rate, twist ratio and swirl velocity on the transient CHF are investigated into details and the widely and precisely predictable correlations of the transient CHF against inlet and outlet subcoolings for the circular tubes with various twisted-tape inserts are given based on the experimental data. The correlations can describe the transient CHFs for SUS304-circular tubes with various twisted-tapes of twist ratios (y = 2.40, 3.37, and 4.45) in the wide experimental ranges of exponential periods (τ = 28.39 ms to 8.43 s) and swirl velocities (usw = 5.04–20.72 m/s) obtained in this work within −26.19% to 14.03% difference. The mechanism of the subcooled flow boiling critical heat flux in a circular tube with twisted-tape insert is discussed.

Proceedings Papers

*Proc. ASME*. ICONE21, Volume 6: Beyond Design Basis Events; Student Paper Competition, V006T16A009, July 29–August 2, 2013

Paper No: ICONE21-15303

Abstract

This study is aimed to clarify transient heat transfer process between the surface of solid and the neighboring helium gas in Very High Temperature Reactor (VHTR) or intermediate heat exchanger (IHX). In this paper a series of platinum heaters with different widths under different pressures inside a circular channel have been tested for forced convection flow. The heat generation rate of the platinum heater was increased with a function of Q 0 exp(t/τ) (where t is time and τ is period of heat generation rate or e-fold time). The heaters were platinum plates with a thickness of 0.1 mm and widths of 2 mm, 4 mm and 6 mm. In the present study, the heat flux, surface temperature, and transient heat transfer coefficients were measured for helium gas passing by horizontal plates under wide experimental conditions such as velocities, pressures and periods of heat generation rate. It was clarified that the heat transfer coefficient approaches the quasi-steady-state when the period is more than around 1 s and it becomes higher when the period shorter than around 1 s. Based on the experimental data, empirical correlations for both quasi-steady-state heat transfer and transient state one at various plate-widths were obtained. It was also found that the heat transfer coefficient becomes higher with the increases of gas pressure.

Proceedings Papers

*Proc. ASME*. ICONE21, Volume 4: Thermal Hydraulics, V004T09A077, July 29–August 2, 2013

Paper No: ICONE21-16174

Abstract

The subcooled pool boiling critical heat flux (CHF) using a 1.2-mm diameter horizontal commercial surface cylinder in water at pressures ranging from atmospheric pressure up to 2063 kPa for a wide ranges of liquid subcoolings from zero to 80 K have been measured to investigate for the mechanisms of subcooled boiling CHF depending on the effects of subcoolings and pressures on the steady and transient CHF. The steady-state CHFs for subcooling were divided into two groups for low and high subcoolings with pressure as a parameter with two different CHF mechanisms resulting from hydrodynamics instability (HI) and heterogeneous spontaneous nucleation (HSN). It was also observed that the measured CHFs resulting from the HI at low pressures in liquid subcoolings as well as the measured CHFs due to HSN at high pressures in high subcoolings respectively on a 1.2-mm diameter of commercial and rough (CS and RS) surface cylinders were approximately same with one another independently of surface conditions. The purpose of the present work is to investigate and compare the heat transfer processes and the vapor behaviors for two mechanisms of subcooled CHFs for the liquid subcooling of 20 K at 101.3 kPa and for the liquid subcooling of 80 K at 690 kPa by mean of photographically. The vapor behavior such as periodic bubble growths and detachments corresponding to HI mechanism at CHF for the liquid subcooling of 20 K at 101.3 kPa and small vapor bubbles existing for some periods on the cylinder corresponding to the HSN mechanism at CHF for the liquid subcooling of 80 K at 690 kPa were investigated.

Proceedings Papers

*Proc. ASME*. HT2013, Volume 1: Heat Transfer in Energy Systems; Thermophysical Properties; Theory and Fundamental Research in Heat Transfer, V001T03A031, July 14–19, 2013

Paper No: HT2013-17227

Abstract

Forced convection transient heat transfer coefficients have been measured for nitrogen gas flowing over a twisted heater due to exponentially increasing heat inputs ( Q 0 exp(t/τ) ). And then, the effect of heater configuration on transient heat transfer by a twisted heater has been investigated comparing to that of the plate heater. In the experiment, the platinum ribbon with a thickness of 0.1 mm and a width of 4.0 mm was used as a test heater. For heat transfer enhancements in single-phase flow, it was twisted at the central part of the heater with an angle of 90 degrees with respect to the upper part of the heater. The heat generation rate was exponentially increased with a function of Q 0 exp(t/τ) . The gas flow velocity ranged from 1 to 4 m/s for the gas temperatures of 313K. The periods of heat generation rate ranged from 46 ms to 17 s. The surface temperature difference and heat flux increased exponentially as the heat generation rate increased with the exponential function. The heat transfer coefficients for twisted heater have been compared to those of the plate heater. They were 24 % higher than those of the plate one. The geometric effect (twisted effect) of heater in this study showed an enhancement on the heat transfer coefficient. It was considered that the heat transfer coefficients are affected by the change in the flow due to swirling flow on the twisted heater. Finally, the empirical correlations for quasi-steady-state heat transfer and transient one have been obtained based on the experimental data.