A CFD investigation was carried out to compare the thermal performance of the 1EHT-1 and 1EHT-2 tubes with a smooth surface tube using R410A at 311K as working fluid. These tubes have enhanced heat transfer area generated by a series of dimples/protrusions and petals distributed over its surface. All the stages of this simulation were conducted using Ansys Fluent. Initially, the physical model of the fluid domain was developed using the Design Modeler module, with an internal tube diameter of 8.32mm, and then imported to the meshing module for the griding process. To ensure accuracy in the results, the mesh average orthogonal quality was kept above 0.7, with the minimum orthogonal quality higher than 0.1. For the numerical simulation, SST k-omega model was used, with Reynolds number ranging from 16000 to 35000. The results of the heat transfer coefficient were validated based on previous experimental work. As expected, at the lowest Reynolds number tested, the heat transfer coefficient for the 1EHT-1 tube was 1097.5 W.K −1 .m −2 , followed by 1058 W.K −1 .m −2 for the 1EHT-2 and nearly 846 W.K −1 .m −2 for the smooth tube. When compared with the experimental results, a good agreement was observed, and the HTC relative error (RE) for all tubes tested was below 10%. It is possible to conclude that the CFD model used here presents as powerful tool to simulate and predict heat transfer with good accuracy, allowing optimization in heat exchangers design and operational parameters.

The most common technique for a surface to have the desired thermo-physical properties or to reflect the desired optical properties is the surface coating. It is used in a wide application area from under the seas to the depths of space. In order for a surface coating to be used, it must have well-defined thermo-physical properties. These properties could be properties that are directly dependent on the material such as heat capacity, heat transfer coefficient and thermal expansion coefficient, as well as material and surface-dependent properties such as absorptivity, reflectivity and transmissivity. Absorptivity is a very important parameter for thermal management of electronics operating under direct solar radiation. Optical methods used to measure the absorptivity are expensive and time consuming methods. In this study, obtaining the absorptivity value of a surface coating with a fast and inexpensive method is investigated. The method consists of two stages; experiment and conduction-based finite element simulation. An aluminum plate with the surface coating to be measured and an aluminum plate with well-known thermo-physical properties without any surface coating are tested in the same environment condition (wind, solar radiation etc.). In the experiment, the time-dependent temperature measurement is taken on both plates separately. Also direct solar radiation and wind speed measurement from the environment are taken. Using these measurements, the FE model is validated and the experiment condition is simulated. The absorptivity of the surface coating is obtained with the iterative solutions made as a result of the analysis test verification studies. The iterative results are presented comparatively with the maximum percent error between simulation and experiment. Consequently, unlike the known methods, the material absorptivity could be measured with a very simple, time efficient and cheap method.

An experimental investigation of shell-side flow condensation heat transfer was performed on advanced three-dimensional surface-enhanced tubes, including a herringbone micro-fin tube and a newly-developed 1-EHT tube. An equivalent plain tube was also tested for performance comparison. All of the test tubes have similar geometry parameters (inner diameter 11.43mm, outer diameter 12.7mm). Tests were conducted using R410A as the working fluid at a condensation saturation temperature of 45 °C, covering the mass flux range of 10–55 kg/(m 2 ·s) with an inlet quality of 0.8 and an outlet quality of 0.1. Experimental results showed that the plain tube exhibits a better condensation heat transfer performance when compared to the enhanced tubes. Moreover, the mass flux has a significant influence on the heat transfer coefficient for shell-side condensation: the condensation heat transfer coefficient of plain tube decreases when the refrigerant mass flux becomes larger, while the heat transfer coefficient of herringbone tube shows a non-monotonic trend and the heat transfer coefficient of the 1-EHT tube gets higher with increasing refrigerant mass flux. Besides, A new prediction model based on the Cavallini’s equation was developed to predict the condensing coefficient of the three test tubes, and the mean absolute error of the improved equations is less than 4%.

This study was performed to investigate the heat transfer and pressure drop of R134A during condensation inside a stainless steel micro-scale enhanced surface tube (EHT tube) and smooth tube. The tests were conducted at a saturation temperature of 45°C, over the mass fluxes range of 100 to 200 kg/m 2 s, the heat fluxes of 14–25 kW/m 2 , an inlet vapor quality of 0.8 and outlet vapor quality of 0.2. The heat length and inner diameter of the tested tube were 2 m and 11.5 mm. The micro-scale enhanced surface tube has complex surface structures composed of dimples and petal arrays background patterns. It can be observed the condensation heat transfer coefficients of the EHT tube is about 1.6–1.7 times higher than that of a stainless steel smooth tube. Enhancement of the EHT tube was achieved due to disruption of the boundary layer, secondary fluid generation, increasing fluid turbulence and heat transfer area. In addition, considering the friction pressure drop, the EHT tube produces the larger friction pressure drop, which is 1.05–1.20 times as compared to the smooth tube. Finally, the performance factors were performed to evaluate the enhancement effect of the EHT tube based on heat transfer coefficient-pressure drop evaluation criteria value ( η 1 ) and heat transfer coefficient-area evaluation criteria value ( η 2 ).

The performance on the air side flow is often limited due to its lower heat transfer coefficient. This work is related to numerical simulation to study the significance of employing delta winglets in flat finned and wavy finned-tube heat exchangers. For this purpose, three-dimensional simulation data and a multi-objective genetic algorithm are employed. The angle of attack (α) of delta winglets and Reynolds number varied from 15° to 75° and 500 to 1300, respectively. Employing delta winglets has increased the heat transfer per unit temperature and per unit volume ( Z ) and the fan power per unit core volume ( E ) for both flat finned and wavy finned-tube heat exchangers. To achieve a maximum heat transfer enhancement and a minimum friction factor, the optimal values of these parameters ( Re and α) are calculated using the Pareto optimal strategy. For this purpose, CFD data, a surrogate model (neural network) and a multi-objective optimization genetic algorithm are combined. Results show that the performance of wavy finned-tube heat exchangers is higher than flat-finned tube heat exchangers which signify the importance of delta winglets in the wavy finned-tube heat exchangers.

A flow boiling heat transfer experimental investigation on the shell-side of a smooth tube and two enhanced tubes (C-EHT, P-EHT) was performed using R410A as working fluid. The outer surface of the C-EHT tube is covered with special fins having a salient height, and the inner surface has a thread structure with the same distribution density. The outer surface of the P-EHT tube has lots of tiny pits formed by high-strength sandblasting and smooth inner surface. The inlet and outlet vapor quality is 0.2 and 0.8 at a saturation temperature of 279K. The flow boiling experimental results show that the evaporation heat transfer coefficient of C-EHT tube is the largest among the tested tubes. However, for the P-EHT tube, its heat transfer coefficient is lower than that of smooth tube. C-EHT tube with special fins has higher heat transfer area, which can also intensify the interfacial turbulence with increasing fins. The reason why the flow boiling heat transfer coefficient of the P-EHT tube is lower than that of the smooth tube is that the diameter of the pit on P-EHT tube is too small to form the nucleation cites, which results in bubble retention under the condition of low mass flux and deteriorates the flow boiling heat transfer performance.

Single-phase and flow condensation experiments were performed using refrigerant R410A in the outer annular region of horizontal enhanced tube with different enhanced surfaces at a saturation temperature of 45°C in the range of mass flux 44.43–102.23kg/(m 2 s). The vapor quality ranges from 0.8 to 0.2. The outer diameters of the tubes are all 19.05mm, but the inner diameters are slightly different due to different surface structures. The surface structures of the three enhanced tubes are fins(EHT1 tube), toothed structures (EHT2 tube) and fine cavities(EHT3 tube) of different sizes and densities. Among them, the EHT3 tube has internal threads. Wilson diagram method was used to determine the enhancement ratio of the water side heat transfer coefficient of EHT3 tube. It was found that the pressure drop increased with the increase of mass flux, while the heat transfer coefficient showed different trends, and the smooth tube was always the lowest of the four tubes. A comprehensive evaluation factor α combining heat transfer enhancement factor ( EF ) and pressure drop penalty factor ( PF ) was defined, in which EHT2 tube (1.38–1.75) was the largest, with strong heat transfer capacity and small pressure drop, so the condensing heat transfer capacity of EHT2 tube was the best.

Dropwise condensation yields higher heat transfer coefficients by avoiding the thermal resistance of the condensate film, seen during filmwise condensation. This work explores further enhancement of dropwise condensation heat transfer through the use of electrowetting to achieve faster droplet growth via coalescence of the condensed droplets. Electrowetting is a well understood microfluidic technique to actuate and control droplets. This work shows that AC electric fields can significantly enhance droplet growth dynamics. This enhancement is a result of coalescence triggered by various types of droplet motion (translation of droplets, oscillations of three phase line), which in turn depends on the frequency of the applied AC waveform. The applied electric field modifies droplet condensation patterns as well as the roll-off dynamics on the surface. Experiments are conducted to study early-stage droplet growth dynamics, as well as steady state condensation rates under the influence of electric fields. It is noted that this study deals with condensation of humid air, and not pure steam. Results show that increasing the voltage magnitude and frequency increases droplet growth rate and overall condensation rate. Overall, this study reports more than a 30 % enhancement in condensation rate resulting from the applied electric field, which highlights the potential of this concept for condensation heat transfer enhancement.

Multi-droplet impact on a thin liquid film covering a heated wall is numerically studied using a three-dimensional model with an implement of a random disturbance subjected to Gaussian distribution. The model can effectively simulate the splashing process after droplets impingement. Results show that simultaneous impact lowers splashing threshold due to cracking of central liquid sheet, while successive impact results in higher splashing threshold for the trailing droplet than the leading one due to radial flow inside the residual film. Single-phase heat transfer coefficient in the impinged region is remarkably higher than the undisturbed film region, caused by flow enhanced convective heat transfer. A heat transfer blind spot situated under the central liquid sheet in simultaneous impact is identified, at which relatively high local temperature is observed and heat is more difficult to be rejected due to flow stagnation there. Cooling of the heated wall in successive impact is mainly achieved by contacting of the trailing droplet with the heated wall, rather than mixing of cold droplets and warm film. This study discusses the effects of Weber number, wall heat flux and droplets horizontal/vertical interval on the heat transfer coefficient.

Additive manufacturing technology and applications have quickly expanded into many industries over the last several years. Improvements in resolution, strength, and material options have helped propel further growth of the industry. This study focuses on an additive manufacturing technology called fused filament fabrication (FFF). FFF involves the extrusion and layer-by-layer deposition of a molten thermoplastic material to create the desired part. One potential new application of FFF is the manufacture of heat exchangers and heat sinks. This study focuses on developing baseline experimental data related to convective heat transfer coefficients over surfaces of commonly used polymers in FFF 3d printing while varying printing parameters. Samples with layer heights (LH) of 0.1 mm, 0.2 mm and 0.3 mm were printed. As the layer height increases, the surface roughness also increased. Sample 1 of LH = 0.1 mm had a roughness of 9.72 μm and at a Reynolds number of 13,200 had a heat transfer coefficient of 72.2 W/m 2 -K and sample 1 of LH = 0.3 mm had a roughness of 28.83 μm and at a Reynolds number of 13,600 had a heat transfer coefficient of 84.6 W/m 2 -K.

This paper presents a discretized enthalpy analysis of a FAME-MLL, NH3 plate-fin heat exchanger (PFHX) with single and two-phase flow. The paper outlines the methodology used to model the PFHX and presents a MATLAB simulation model, which uses the NIST REFPROP database for thermo-physical property evaluation. The paper presents analysis for a staged single-phase, two-phase PFHX architecture. Plots of the heat transfer coefficients, overall heat transfer coefficient, specific enthalpy and heat flux variation over the length of the PFHX are provided. The mass, volume, area, effectiveness-number of transfer units rating for each PFHX are given.

Jet impingement is a cooling technique commonly employed in combustor liner cooling and high-pressure gas turbine blades. However, jets from upstream impingement holes reduce the effectiveness of downstream jets due to jet deflection in the direction of crossflow. In order to avoid this phenomenon and provide an enhanced cooling on the target surface, we have attempted to come up with a novel design called “crossflow diverters”. Crossflow diverters are U-shaped ribs that are placed between jets in the crossflow direction (under maximum crossflow condition). In this study, the baseline case is jet impingement onto a smooth surface with 10 rows of jet impingement holes, jet-to-jet spacing of X/D = Y/D = 6 and jet-to-target spacing of Z/D = 2. Crossflow diverters with thickness ‘t’ of 1.5875 mm, height ‘h’ of 2D placed in the streamwise direction at a distance of X = 2D from center of the jet have been investigated experimentally. Transient liquid crystal thermography technique has been used to obtain detailed measurement of heat transfer coefficient for four jet diameter based Reynolds numbers of 3500, 5000, 7500, 12000. It has been observed that crossflow diverters protect the downstream jets from upstream jet deflection thereby maximizing their stagnation cooling potential. An average of 15–30% enhancement in Nusselt number is obtained over the flow range tested. However, this comes at the expense of increase in pumping power. Pressure drop for the enhanced geometry is 1–1.5 times the pressure drop for baseline impingement case. At a constant pumping power, crossflow diverters produce 9–15% enhancement in heat transfer coefficient as compared to baseline smooth case.

The presented study numerically investigated the heat transfer characteristics of supercritical hydrocarbon fuel RP-3 in a vertical tube under overweight conditions with gravitational accelerations from 1g to 5g. The model was simplified as a vertical tube with the diameter of 1.8mm and the length of 250mm. Constant heat flux was applied to the wall, varying from 200kW/m 2 to 700kW/m 2 . Variations of wall temperature and heat transfer coefficient under overweight conditions were obtained by simulation. The dimensionless buoyancy and thermal acceleration under different conditions were analyzed. The results show that the heat transfer is normal at low heat flux, while two types of heat transfer deterioration were observed in both upward and downward supercritical flow at high heat flux. The heat transfer coefficient of downward flow is generally higher than upward flow, and the difference between them becomes larger with the increase of gravitational acceleration. At high heat flux, when bulk temperature reaches the pseudo-critical temperature, the thermal acceleration will increase by 50% leading to the deterioration of heat transfer. However, after the pseudo-critical point, both buoyancy force and thermal acceleration decrease to negligible. The rise in gravitational acceleration enhances buoyancy force, but has no impact on the thermal acceleration. Based on the numerical analysis, two different criterion, Bo* and Kv, for supercritical RP-3 are obtained to present the influence of buoyancy and thermal acceleration.

The performance of a falling-film heat exchanger is strongly linked to the surface characteristics and the heat transfer processes that take place over the tubes. The primary aim of this numerical study is to characterize the influence of surface wettability of tubes on the falling film flow mode and its associated surface heat transfer. Surface wettability is generally characterized by the contact angle and, in this study, the wettability characteristics ranged from superhydrophilic to a superhydrophobic tube surface. The dynamic motion of the triple contact line connecting the solid, liquid and gas phases over the tube surface is replicated with the help of the Kistler’s dynamic contact angle model. The volume of Fluid (VOF) simulations was carried out for the flow and heat transfer of liquid flow over horizontal tubes with different surface wettabilities. The mass flow rate is such that the flow is in the jet mode where the liquid flows in the form of jets in between the horizontal tubes. This corresponds to a liquid mass flow rate per unit tube length of 0.06 and 0.18 Kg/m-s, under which the inline and staggered jets modes of flow are observed. Under the two flow rates and different surface wettabilities, the liquid flow hydrodynamics over the tube surfaces was explored in terms of the liquid film thickness, the contact areas (solid-liquid and liquid-air) between the different phases, and the heat transfer coefficient. The axial resistance imposed by the increasing contact angle tends to inhibit the extent of the liquid spreading over the tube surface and this, in turn, influences the liquid film thickness and the wetted area of the tube surface. A significant decrement in the heat transfer rate from the tube surfaces was observed as the equilibrium contact angle ranged from 2° to 175°. Heat transfer characteristics were quantified over a wide range of contact angles for the two mass flow rates. Fluid recirculations were observed in the liquid bulk which had a major impact on the heat transfer distribution over the tube surface.

Performance enhancements in heat exchanger design and manufacturing have been achieved over the past several decades through a combination of improved thermal-hydraulic modeling and experimentation tools, enhancements in material formulations and associated property characterizations, and new manufacturing methods. Most recently, Additive Manufacturing (AM) methods have matured sufficiently that they are now being considered as realizable heat exchanger fabrication methods. More complex, compact, and efficient designs can be achieved with AM methods that could not be easily obtained through more traditional manufacturing techniques. This study expands upon a previous work [1] in which an optimized twisted tube shell and tube stainless steel heat exchanger was designed, analyzed, and fabricated with a Direct Metal Laser Sintering (DLMS) AM method. In that study, the twisted tube heat exchanger performance was a considerable improvement over that of a traditional straight tube shell and tube heat exchanger. In the present study, the AM twisted tube heat exchanger was subjected to thermal-hydraulic tests to measure its performance and to identify any necessary refinements to the previous CFD model. For the conditions used in this study, the experimental data will show how the previous CFD model over-predicted the twisted tube heat exchanger’s heat transfer rate of 2,297 W and under predicted its overall heat transfer coefficient of 1,008 W/m 2 /K. Interrogation of the CFD model found that this discrepancy was due to the utilization of a k-ε turbulence model. Once this turbulence characterization was replaced with a more suitable shear transport model, the numerical predictions and experimental measurements of total heat transfer rate and overall heat transfer coefficient were in very close (∼10%) agreement. When combined with the previous study, this current work reveals how a complex, twisted tube shell and tube heat exchanger can be designed with existing CFD modeling tools and efficiently manufactured with current AM technologies to significantly improve its performance over a more traditionally manufactured straight tube version of the heat exchanger.

In this paper a numerical investigation on mixed convection in confined slot jets impinging on a porous media is accomplished. The working fluids are pure water or Al2O3/water based nanofluids and a single-phase model approach has been adopted in order to describe their behavior. A two-dimensional configuration is analized and different Peclet numbers and Rayleigh numbers are considered. The thermal non-equilibrium energy condition is assumed to execute two-dimensional simulations on the system. The examined foams are characterized by distinct values of pores per inch, PPI, equal to 5, 10, 20 and 40. The particle volume concentrations range from 0% to 4% and the particle diameter is equal to 30 nm. The target surface is heated by a constant temperature value, calculated according to the value of Rayleigh number. The distance of the target surface is five times greater than the slot jet width. The aim consists into study the thermal and fluid-dynamic behaviour of the system. Results show increasing values of the convective heat transfer coefficients for increasing values of Peclet number and nanoparticle concentration. Furthermore, the heat transfer coefficient presents a different behavior at varying PPI numbers for different Peclet numbers.

Metal matrix composites (MMCs) can be manufactured by infiltrating a melting matrix alloy into hard powders — such as silicon carbide and tungsten carbide — loaded in a graphite mold and quenched to achieve a specific quenching temperature profile for proper solidification. Water quench is a widely used quenching technique within the aluminum and steel industry. It is more common to apply numerical simulation to optimize process parameters and help improve product quality, which depends upon reliable boundary conditions (e.g., heat flux or heat transfer coefficient); however, heat transfer coefficient changes with surface temperature and water flow rate. Moreover, the heat transfer coefficient in the discussed manufacturing process was never quantified. A combined experimental and simulation method to investigate heat transfer coefficient of the external surface of the graphite mold associated with water quenching is proposed. Firstly, the heat flux from the graphite mold is measured, which varies with water flow rate, mold surface temperature, nozzle arrangement, and water flow pattern. Without modifying the hardware design, this study focuses on the effects of water flow rate and mold surface temperature on surface heat flux. Secondly, the temperature distribution within the mold is used to inversely determine the heat transfer coefficient by solving an inversed optimization problem.

Offset strip fin channels promote higher heat transfer rate than plain fins due to the periodic disruption of boundary layers in the offset channels and downstream flow mixing. The enhanced convection heat transfer in laminar air flows (50 < Re < 2000; Pr = 0.7) is computationally investigated. By considering the fin height h to be much larger than the inter-fin spacing s (or s/h <<1) which is representative of many practical applications, a two-dimensional flow channel is modeled. With thin fins ( t/s ) < 20%, the effect of fin offset length l (1 ≤ l/s ≤ 35) on the convective enhancement is explored. Higher heat transfer coefficients are found to be obtained with shorter fin lengths and higher flow Reynolds number Re . This effect, however, diminishes for low Re and very large fin offset length and performance asymptotically approaches that of plain rectangular fin channels. The frictional loss also increases correspondingly, and is primarily due to flow acceleration and boundary-layer disruption. This penalty is offset by a 2.5–3 times higher heat transfer rate capacity for fixed pressure drop condition relative to plain fins. For a fixed heat transfer rate application, offset-strip fins are shown to yield 40%–50% smaller surface area cores.

Experimental condensation heat transfer data from our previous studies were collected to find a suitable heat transfer correlation for three-dimensional surface enhanced heat transfer tubes. Dimples/protrusions and petal arrays make up the enhanced surface of the 1EHT tube, while longitudinal grooves and dimples constitute that of the other two 2EHT tubes. Working fluids investigated includes three refrigerants R22, R32, R410A and three enhanced tubes have a same nominal inner diameter of 12.7mm. Due to the unique and complex surface structure of three EHT tubes, a constant is proposed and utilized for each EHT tube to estimate the impact of enhanced surfaces on interfacial turbulence, boundary layer disruption, flow separation and secondary flow generation. In addition, the Cavallini correlation modified with the constant can predict almost all the data points within a ±5% error band, which can be a good example for how to predetermine the heat transfer coefficient of three-dimensional enhanced tubes with a unique surface.

The rapid development of Additive Manufacturing (AM) technologies has provided engineers with new methods to design and fabricate complex mechanisms. AM offers unique methods to allow for integration and simplification of components, reduced manufacturing time, fabrication of complex-shaped objects, improvements upon existing designs, and extending the creative design space which engineers rely on for ingenuity. For many applications, heat exchanger performance can be improved by reducing its size, increasing the overall heat transfer coefficient and surface area, and making more efficient use of the mechanical structure for heat exchange. Traditional manufacturing often limits or prohibits many of these enhancements due to increased manufacturing and assembly costs. This study explored using AM to design and fabricate a compact twisted tube stainless steel shell and tube heat exchanger that would improve upon all of the features just mentioned. This paper discusses the design of the heat exchanger and the AM technique used to fabricate a prototype. The manuscript will show via CFD analyses, how the heat transfer area of the unit was improved 18% and the overall heat transfer coefficient as increased by 40% over a traditional round tube heat exchanger with an identical footprint. Further, the study will show how AM was leveraged to combine five manufacturing steps into one to fabricate a prototype, fully functional twisted tube heat exchanger.