Two-phase Heat Transfer Coefficient Research Articles

Numerical investigation of bubble dynamics and flow boiling heat transfer in cylindrical micro-pin-fin heat exchangers

Micro-pin-fin evaporators are a promising alternative to multi-microchannel heat sinks for two-phase cooling of high power-density devices. Within pin-fin evaporators, the refrigerant flows through arrays of obstacles in cross-flow and is not restricted by the walls of a channel. The dynamics of bubbles generated upon flow boiling and the associated heat transfer mechanisms are expected to be substantially different from those pertinent to microchannels; however, the fundamental aspects of two-phase flows evolving through micro-pin-fin arrays are still little understood. This article presents a systematic analysis of flow boiling within a micro-pin-fin evaporator, encompassing bubble, thin-film dynamics and heat transfer. The flow is studied by means of numerical simulations, performed using a customised boiling solver in OpenFOAM v2106, which adopts the built-in geometric Volume of Fluid method to capture the liquid–vapour interface dynamics. The numerical model of the evaporator includes in-line arrays of pin-fins of diameter of 50μm and height of 100μm, streamwise pitch of 91.7μm and cross-stream pitch of 150μm. The fluid utilised is refrigerant R236fa at a saturation temperature of 30 °C. The range of operating conditions simulated includes values of mass flux G=500–2000kg/(m2s), heat flux q=200kW/m2, and inlet subcooling ΔTsub=0–5K. This study shows that bubbles nucleated in a pin-fin evaporator tend to travel along the channels formed in between the pin-fin lines. Bubbles grow due to liquid evaporation and elongate in the direction of the flow, leaving thin liquid films that partially cover the pin-fins surface. The main contributions to heat transfer arise from the evaporation of this thin liquid film and from a cross-stream convective motion induced by the bubbles in the gap between the cylinders, which displace the hot fluid otherwise stagnant in the cylinders wakes. When the mass flow rate is increased, bubbles depart earlier from the nucleation sites and grow more slowly, which results in a reduction of the two-phase heat transfer. Higher inlet subcooling yields lower two-phase heat transfer coefficients because condensation becomes important when bubbles depart from the hot pin-fin surfaces and reach highly subcooled regions, thus reducing the two-phase heat transfer.

Improved flow boiling performance and temperature uniformity in counter-flow interconnected microchannel heat sink

Microchannel flow boiling is one of the most effective solutions to the problem of heat dissipation caused by high power electronic devices. However, inherent drawbacks such as low critical heat flux due to dry-out of the liquid film near the channel exit, relatively high two-phase flow pressure drop, severe temperature nonuniformity and boiling instability at high heat flux still prevent widespread commercial application. In this study, based on the idea of uniform vapor quality along the flow direction, a copper Counter-Flow Interconnected Microchannel (CFIM) was proposed to address the main issues currently faced by two-phase microchannel heat sink. A comprehensive comparative study of CFIM, Co-Current Microchannel (CCM) and Counter-Flow Microchannel (CFM) was carried out from the aspects of boiling stability, boiling heat transfer characteristics and temperature uniformity. Three slots of 0.2 mm, 0.5 mm and 0.8 mm in width, denoted as IM0.2, IM0.5 and IM0.8, respectively, are considered. Experiments are conducted at mass fluxes of 291–618 kg/m2·s, saturation temperature of 40 °C and effective heat fluxes of 3–293 W/cm2, using the dielectric fluid R1233zd(E) as the working fluid. The results show that the CHF and average two-phase heat transfer coefficient (HTC) of CFIM are increased by 40.6 ∼ 100.5 % and 70.4–83.6 %, respectively, while the two-phase pressure drop is decreased by 17–32.6 % compared to CCM. The design of the counter-flow interconnected structure can manipulate the void fraction and two-phase flow pattern, resulting in a near uniform void fraction and flow pattern. The structure of these connecting slots promotes nucleate boiling and fluid mixing of neighboring channels. More importantly, the flow boiling instability and temperature non-uniformity in CFIM are well suppressed. In addition, IM0.2 has higher heat dissipation than IM0.5 and IM0.8 due to enhanced bubble nucleation, reducing superheat requirements and periodic rewetting to the center of the channel. The results of this research reveal that utilizing a microchannel heat sink with a counter-flow interconnected configuration can effectively improve heat transfer performance during flow boiling. Furthermore, it helps to mitigate issues such as flow boiling instability and temperature non-uniformity.

Experimental study on flow boiling of HFE-7100 in rectangular parallel microchannel

With the rapid development of microelectronic technology, the integration and power of chip are increasing. Heat dissipation with high heat flux in limited space has become a bottleneck restricting the efficient and stable operation of the microelectronic devices. Flow boiling in microchannel heat sink is one of the most essential candidates for solving this problem. It has been shown that remarkable high heat transfer performance can be achieved through the liquid-to-vapor change process, which can dissipate a large amount of heat from a small area. In addition, dielectric fluorinated fluids, such as HFE-7100, HFE-7200, and FC-72, are especially suitable for cooling microelectronic devices, because of their excellent safety and environmental characteristics. However, dielectric fluorinated fluids have poorer thermophysical properties than water. Thus, the flow boiling heat transfer characteristics of dielectric fluorinated fluids can be different from those of water. In this work, flow boiling heat transfer and flow characteristics of HFE-7100 in a rectangular parallel microchannel are investigated. The tests are conducted at mass fluxes from 88.9 to 277.8 kg·m<sup>–2</sup>·s<sup>–1</sup>, inlet subcooling temperature from 20.5 to 35.5 ℃ and effective heat flux from 12 to 279 kW·m<sup>–2</sup> at nearly atmospheric pressure. The effects of mass flux, inlet subcooling temperature, effective heat flux and vapor quality are examined and analyzed. Additionally, flow visualization is also obtained to explain the heat transfer mechanism during the experiments. The results show that the boiling hysteresis is observed for HFE-7100 at low inlet subcooling temperature, and the increasing inlet subcooling temperature and mass flux can delay the onset of nucleate boiling. The increases of inlet subcooling temperature and mass flux can enhance the two-phase heat transfer coefficient. And the two-phase heat transfer coefficient is significantly dependent on the inlet subcooling temperature in the slug flow, while it is significantly dependent on the mass flux in the annular flow. The two-phase pressure drop increases drastically as the effective heat flux increases. And the two-phase pressure drops with different mass fluxes at constant vapor quality are obviously different between the slug flow and the annular flow. Furthermore, the experimental data are compared with four predicted values of the literature. It is found that the correlation of Lockhart has the best statistical agreement with an MAE of 19.6% and over 85% of points in the deviation bandwidth of ±30%. The results in this paper give valuable theoretical guidance for designing and optimizing heat dissipation equipment for microelectronic devices. By utilizing HFE-7100 as the coolant and microchannel heat sinks in flow boiling, it is possible to enhance the stability and reliability of the electronic devices. Additionally, the heat transfer performance associated with different heat fluxes can be improved by regulating the inlet subcooling and mass flow rate. Finally, the two-phase pressure drop correlation proposed by Lockhart can be employed to predict the pump power for heat dissipation equipment.

Enhanced thermo-hydraulic performance with two-phase flow boiling in wavy microchannel heat sink – A numerical investigation

This article demonstrates that using a wavy microchannel instead of a smooth microchannel can enhance thermal and hydraulic performance in both single and two-phase flow. Various performance parameters such as Nusselt number (Nu), friction factor (f), performance factor (PF), and enhancement factor (EF) are estimated for different wavy microchannels with varying waviness and amplitudes. The two-phase heat transfer coefficient (HTC) varies significantly with heat flux, particularly due to nucleate boiling. The heat transfer improvement with a wavy microchannel is evident with the highest values of the Nusselt number ratio (Nutp/Nusp = 2.35), PFwavy = 1.4, and EF = 1.88 achieved at specific operating conditions (heat flux = 100 W/cm2, mass flux = 188 kg/m2s, amplitude = 0.1 mm, wavelength = 1 mm). However, as mass flux, heat flux, and amplitude increase and wavelength decrease, the Nusselt number ratio Nutp/Nusp, PF, and EF decline due to the pressure drop penalty outweighing the benefits of heat transfer enhancement. The article also explores different designs of wavy microchannels for mitigating hot spots, including one with a decreasing wavelength along the flow direction and another with a shorter wavelength in the middle. However, these designs are more suitable for single-phase flow.

Experimental study of flow boiling characteristics of open microchannels with elliptical cavities and elliptical ribs

Open microchannel heat sinks (O-MCHS) stand out among many thermal management technologies due to their extra open gaps that reduce pressure drop and mitigate flow instability. Recently, there has been a growing trend of incorporating microstructures onto microchannels in order to enhance the hydrothermal performance of heat sinks. However, the heat transfer characteristics and bubble behavior of O-MCHS with microstructures are still rarely studied. In this study, open copper-based microchannel heat sinks with elliptical cavities and elliptical central ribs (O-EC-ECR) and elliptical cavities and elliptical side ribs (O-EC-ESR) were designed and manufactured; the flow patterns, heat transfer performance and pressure drop were systematically studied and compared with those of rectangular open microchannel heat sink (R O-MCHS). The working fluid was degassed DI water and the experiments were performed with the inlet subcooling of 75 ℃ at efficient heat fluxes of 668.00–2416.16 kW/m2 under various mass fluxes ranging from 421 to 801 kg/m2·s. The experimental results demonstrated that the O-EC-ECR exhibited the best two-phase heat transfer performance among three structures at all mass fluxes. The average two-phase heat transfer coefficient of O-EC-ECR was increased by 12.2 % compared to R O-MCHS at the mass flux of 801 kg/m2·s. The average wall temperature of O-EC-ECR was found to be 10.1 ℃ lower than that of R-O-MCHS and 6.7 ℃ lower than that of O-EC-ESR at the mass flux of 801 kg/m2·s. This should be attributed to the dramatically increased convective heat transfer area as well as the separating and breaking effect of the central ribs on the bubbles, which was manifested by the increase in heat transfer coefficient and critical heat flux (CHF). In addition, the central ribs improved the flow boiling stability by effectively suppressing the generation of confined bubbles and the formation of slugs. The arrangement of the side ribs and cavities made bubbles easier to condense, which facilitated the mixing of hot and cold fluid. At low to medium mass fluxes, the O-EC-ESR only showed superior heat transfer performance during the bubbly flow dominated nucleate boiling regime. Whereas high mass flux can improve the boiling heat transfer performance; the average heat transfer coefficient of O-EC-ESR was increased by 6.1 % compared to R O-MCHS. The average pressure drop of O-EC-ESR was decreased by 21.8 % compared to R O-MCHS and 25.6 % compared to O-EC-ECR at the mass flux of 801 kg/m2·s.

Statistical and machine learning applied to the universal consolidated database to predict heat transfer coefficient in flow condensation

Due to complex physical mechanisms, and a general lack of accurate modeling techniques, two phase convective heat transfer is hard to characterize. In this article we show the general effectiveness of statistical and machine learning techniques for predicting two-phase heat transfer coefficients. This is a comprehensive study that: performs a full exploratory data analysis (EDA) to determine the range by which we may apply these models, derives an optimal data set as a basis for predicting heat transfer coefficient, tests all models on an independent data set, and assesses model performance on multiple metrics. We find that random forest methods, gradient boosted machines, and multi-layer perceptrons (deep neural networks) perform the best in predicting heat transfer coefficient on the given data set. We find that in general, the increased data base predicts better due to the inclusion of a few important thermophysical and flow properties.

Thermal-hydraulic characterization of R513A during flow boiling inside a 6.0 mm horizontal tube, comparison with R134a and development of a new correlation

This paper presents two-phase heat transfer coefficient and pressure drop data of refrigerant R513A, a new azeotropic mixture conceived as possible alternative to R134a for medium temperature small-size refrigeration systems. All the experiments were performed in a commercial horizontal stainless-steel tube having an internal diameter of 6.0 mm and an outer diameter of 8.0 mm. The channel heating was obtained through DC current and Joule effect. The effect of the main operating parameters in terms of mass flux (from 150 to 500 kg/m2s), heat flux (from 5.0 to 40 kW/m2), saturation temperature (from 30 to 50 °C) and vapor quality (from the onset of boiling up to the dry-out occurrence) was analyzed and discussed for the heat transfer coefficient values, finding that both convective and nucleate boiling were significant contributions. The same ranges of mass flux and saturation temperature were also applied to frictional pressure gradient trends with vapor quality, taken in adiabatic conditions. The thermo-hydraulic performances were then compared with those of R134a, obtaining lower heat transfer coefficients and very similar pressure gradients. Finally, the collected experiments were assessed with values predicted from the most quoted correlations available in literature. A new composite method for nucleate-dominant or convective-dominant mechanisms was proposed for the evaluation of the heat transfer coefficient, with a mean absolute error of 25.3%, whereas the frictional pressure gradient values were well predicted with the Friedel correlation, that provides a mean absolute error of 13.8%.

Heat transfer model for downward two-component two-phase flows in inclined pipes

Downward two-component two-phase flows are commonly encountered in many chemical engineering applications. Accurate prediction of the heat transfer coefficient is of great importance to the design and operation of heat transfer systems. The mechanism of two-phase heat transfer is complicated because of the complex interactions between gas and liquid phases. Therefore, this study was aimed at developing a robust and theoretically supported heat transfer coefficient correlation for downward two-component two-phase flow in inclined pipes with the extended Chilton-Colburn analogy. First, an extensive literature review was performed to collect more than 1700 experimental data for downward two-phase heat transfer coefficients in inclined pipes and 10 heat transfer coefficient correlations. Poor predictive performance of the existing correlations was identified using the whole collected database. Then, the dependence of the heat transfer coefficient on void fraction and two-phase multiplier was analyzed using the extended Chilton-Colburn analogy. The dependence of the heat transfer coefficient on the pipe inclination angle was discussed in depth. The exponents in the newly-developed heat transfer coefficient correlation were correlated in terms of the pipe inclination angle. A robust and semi-theoretically supported correlation was developed for predicting the heat transfer coefficient of downward two-component two-phase flows in inclined pipes. The mean relative deviation (bias), mrel, and the mean absolute relative deviation (random uncertainty), mrel, ab, were −4.54 % and 13.8 %, respectively. The new heat transfer coefficient correlation will be of practical importance in understanding the two-component two-phase heat transfer characteristics.

Influence of system pressure on flow boiling in microchannels

The effect of pressure on the flow boiling characteristics of HFE-7200 between 1.0 to 2.0 bar was investigated in a parallel microchannel heat sink (Dh = 0.48 mm, 44 channels). The mass flux and subcooling degree were kept constant at 200 kg/m2 s and 10 K respectively, while heat flux ranged from 26.1 – 160.7 kW/m2. Increasing system pressure decreased the vapour density ratio, thus leading to reduced pressure drop, increased bubble generation frequency and two-phase heat transfer coefficients in the system, though this may not be apparent at low superheat degrees at higher pressures. Smaller bubble diameters were observed at higher pressures and resulted in a delay in flow regime transition to slug flow, which is prone to flow reversal. The experimental work showed promising means to manage flow instabilities and enhance heat transfer performance in two-phase microchannel systems for HFE-7200.

Irregular-shaped nucleated bubbles induced enhancement of subcooled flow boiling heat transfer in leaf vein inspired three-tiered open microchannels (LTOMCs)

Nature is an important source of inspiration for developing desired structures for various industrial applications. The efficacy of water transport in leaf vein systems provides important insights for the structural design of microchannels. In addition, open microchannels (OMCs) have been reported to effectively reduce the two-phase pressure drop and suppress flow instabilities by providing extra space for bubble and vapor expansion. Thus, the synergistic cooperation of open microchannels with leaf-vein-like structures is expected to alter the flow patterns and improve the heat transfer performance in the flow boiling process. In this work, two leaf-vein inspired three-tiered open microchannels (LTOMC) with exactly the same structure but opposite orientations of vein-like channels, i.e., LTOMC-Ⅰ and LTOMC-Ⅱ, were manufactured, respectively. Flow boiling experiments were conducted at the inlet temperature of 25 °C under different mass fluxes. It was observed that the flow patterns of the LTOMC were divided into bubbly flow, slug flow, reverse and rewet flow; the shapes and movement trends of nucleated bubbles were mainly dominated by the vein-like structures. The required wall superheat of the onset of nucleated boiling for LTOMCs was 2–6 °C lower than ROMC; the one for LTOMC-Ⅱ was 2–3 °C lower than LTOMC-Ⅰ at various mass fluxes. The average two-phase heat transfer coefficient of LTOMC-Ⅱ was increased by 83.2% compared to ROMC at the mass flux of 234 kg/m2·s; the one of LTOMC-Ⅱ was 54.1% higher than LTOMC-Ⅰ. The pressure drop of three OMCs was comparable; however, the onset of two-phase flow instability occurred earlier for ROMC at the same heat flux. Besides, the liquid rewetting capability of ROMC was easier to get deteriorated at ultra-high heat flux. This work offers an avenue for structural optimization of microchannel heat sinks to enhance heat transfer performance without compromising the pressure loss.

Experimental investigation on flow boiling characteristics in offset strip fin channels under different sloshing conditions

In order to reveal the effects of sloshing movement on the performance of the plate-fin heat exchangers applied in floating liquefied natural gas platforms and environment control system of aircrafts, the flow boiling characteristics in offset strip fin channels of plate-fin heat exchangers under different sloshing conditions were investigated experimentally, covering rolling, pitching and swaying movements. The results show that, the rotational type movements enhance the transient heat transfer coefficient by a maximum of 21.1% at low vapor quality condition and weaken the transient heat transfer coefficient by a maximum of 27.7% at high vapor quality condition, while the transitional type movements enhance the heat transfer at high vapor quality. Fast Fourier transform method was used in the spectrum analysis of flow instabilities, and there are two peak frequencies caused by the inlet fluctuations, the distribution of the liquid phase and the enhanced turbulence. The time-average factors were proposed to evaluate the average heat transfer performance during the sloshing period. The ranges of sloshing time-average factors for rolling, pitching, and swaying conditions are 0.861 ∼ 1.079, 0.923 ∼ 1.061, 0.834 ∼ 1.064 and 0.994 ∼ 1.090, respectively. As the mass flux and sloshing period increase, the time-average factors trend to 1.0, and increase at the transitional sloshing movement. With the increasing amplitude, the time-average factors increase from 1 to a maximal value of 1.093. The correlations to predict the time-average two-phase heat transfer coefficients under rolling, pitching, swaying, and heaving conditions for offset strip fins have been developed, and the deviations are within ± 10 %.

Experimental investigation on flow boiling of HFE-7100 in a microchannel with pin fin array

An experimental investigation on flow boiling heat transfer and pressure drop characteristics in a microchannel with an array of in-line circular pin fins was carried out. The tests with HFE-7100 as a working fluid were conducted at mass fluxes from 189 to 374 kg/m2∙s, inlet subcooling temperatures from 20.4 to 34.4 °C and effective heat fluxes from 17.4 to 239.3 kW/m2 near atmospheric pressure. The effects of mass flux, inlet subcooling temperature, heat flux and vapor quality were examined and analyzed. Flow visualization was also obtained during the experiments to explain the heat transfer mechanism. The boiling hysteresis was observed, which may be attributed to the low surface tension of HFE-7100. The experimental data were compared with various published data of dielectric fluorinated fluids, and many well-known correlations of pressure drop and two-phase heat transfer coefficient for pin–fin microchannels. The improved correlations of pressure drop and two-phase heat transfer were proposed, which demonstrated good predictions for both experimental data of the present and previous studies.

Interface Capturing Flow Boiling Simulations in a Compact Heat Exchanger

Abstract High-fidelity flow boiling simulations are conducted in a vertical mini channel with offset strip fins (OSF) using R113 as a working fluid. Finite-element code PHASTA coupled with level set method for interface capturing is employed to model multiple sequential bubble nucleation using transient three-dimensional approach. The code performance is validated against experiments for a single nucleation site in a vertical rectangular channel. To assess code performance, a study on the bubble departure from the wall in a mini channel with OSF is carried out first. Contributions from the microlayer are not considered due to low heat flux values applied to the channel (1 kW/m2). The influence of surface characteristics, such as contact angle and liquid superheat on bubble dynamics, is also analyzed as well as the local two-phase heat transfer coefficient. For higher void fractions, two conical nucleation cavities are introduced in the same channel with OSF. Observed bubble characteristics (departure diameter, bubble departure frequency) are evaluated and bubble trajectories are presented and analyzed. The local heat transfer coefficient is then evaluated for each simulation. The results show approximately a 2.5 time increase in the local heat transfer coefficient when the individual bubbles approach the wall. With smaller bubble nucleation diameters, the heat transfer coefficient can increase by up to a factor of two. Thus, the current work demonstrates the flow modeling capability of the boiling phenomena in complex geometry with OSF.

Zeotropic mixtures study in plate heat exchangers and ORC systems

Based on the known scientific literature, there are no correlations in the scientific literature to predict the two-phase heat transfer coefficients of Hydrofluoroethers pure fluids and zeotropic mixtures used in plate heat exchangers. This work has allowed the development of evaporation and condensation correlations for such fluids. Correlations based on dimensionless numbers and working fluids critical properties gave promising results, with a prediction of the two-phase heat transfer coefficient around 15% for heat source and 50% for cooling source which led respectively to a prediction of 4% and 6% for the global thermal power exchanged. The behavior of the organic Rankine cycle heat sources and the working fluids used within was also been studied. For this purpose, the influence of the heat sources flow rates variation on the performance of the hot and cold exchangers was analyzed. Results obtained showed a similar behavior and performance variation of the heat exchangers both when using pure fluids and zeotropic mixtures. Zeotropic mixtures working fluids in ORC system are often supposed in the literature to significantly increase the performance of this thermodynamic cycle due to better heat transfer at the hot source. Nevertheless, the great majority of these studies are numerical and do not take into account the interactions and system effects existing within the thermodynamic cycle when using a mixture instead of a pure fluid. This work has allowed to highlight this complexity and to provide a better understanding of the use of zeotropic mixtures.

Development of a new correlation for pre-dry out evaporative heat transfer coefficient of R290 in a microchannel

Although many correlations have and are being developed for two-phase heat transfer coefficient correlation in microchannels, none has the desired accuracy yet. This paper presents prospective improvements in the convective heat transfer coefficient correlation accuracy for the dry-out conditions; when the heat transfer coefficient drops at a certain vapor quality. An accurate heat transfer coefficient forecast is needed to minimize over or under design, conserve energy and material, and maximize the performance. R290 has been reported to be more energy efficient than R22. A new correlation relevant for R290 across 831 sets of experimental data points in a microchannel was generated by optimizing six variables in the nucleate boiling suppression factor, S, and force convective factor, F, of a selected superposition type correlation. The new correlation was optimized for saturation temperature ranging between 5 and 25 °C, diameter ranging between 1.0 and 6.0 mm, heat flux ranging between 2.5 and 60 kW/m2, and mass flux ranging between 50 and 500 kg/m2s. The MAE was reduced from 21.84 to 17.02% for pre-dry out data. The new correlation may be utilized to estimate the heat transfer coefficient of R290 in heat transfer analysis in a microchannel under the investigated operating conditions. The optimization approach utilized here has the potential of continuously improving the MAE of any heat transfer correlation for any refrigerants considered.

Flow condensation heat transfer of propane refrigerant inside a horizontal micro-fin tube

Due to the phase-out of CFCs and HCFC refrigerants, propane is a nature refrigerant occurring substance produced by natural gas production and oil refining. As a result, propane has a higher latent heat and lower density than conventional refrigerants while maintaining a comparable saturation pressure and thermal conductivity. So, propane refrigerant is already widely used in domestic fridges and freezers for many years. However, propane’s operating pressures and temperatures are well suited for air conditioning equipment, including chillers. This study investigates the contributions of different heat transfer mechanisms in two-phase flow condensation heat transfer coefficients for propane inside a 6.3-mm ID micro-fin copper tube. Data were collected through an experiment with a two-phase flow condensation. Measurements were taken at different refrigerant mass fluxes from 100 to 300 kg/m2s and heat fluxes from 3 to 9 kW/m2. In addition, the experiments investigated effect of vapor quality, mass flux, and heat flux on the heat transfer coefficients. Finally, a new heat transfer coefficient correlation was developed based on the experimental data with good agreement.

Experimental study of flow boiling characteristics in minigap channels over a wide heat flux range

The flow boiling characteristics in minigap channels were investigated by observing the two-phase flow patterns, measuring the two-phase pressure drops, and calculating the two-phase heat transfer coefficients and critical heat fluxes in minigap channels with heights of 0.5–2 mm. The working fluid was deionized water with mass fluxes of 200–400 kg m−2 s−1. The results show that bubbly flow, sweeping flow, churn flow, and churn-annular flow occur in sequence with increasing heat fluxes. An unstable two-phase flow regime was observed during the sweeping flow. The pressure drop varied in three regimes that were related to the flow patterns. The maximum pressure drops are 12.2, 3.8 and 1.6 kPa, respectively in 0.5 mm, 1 mm and 2mm channels at 400 kg m−2 s−1. The effect of gap height on the heat transfer coefficient first increases and then decreases with increasing heat flux. The heat transfer coefficients of all channels are at a high level when the heat flux is high. The maximum heat transfer coefficients are 7.3, 6.9 and 5.8 W cm−2 K−1 in the 0.5 mm, 1mm and 2mm channels at 400 kg m−2 s−1, which occurred before critical condition. The critical heat flux increased with increasing mass flux and gap height. In addition, a pressure drop correlation was developed to more accurately predict the pressure drop in microgap channels with an accuracy of 10.8%. The Liu-Winterton correlation and the Shah correlation give the best predictions for the subcooling boiling heat transfer coefficient. The Sun-Mishima correlation and the Kandlikar correlation provide the most accurate predictions for the saturated boiling heat transfer coefficients.

Experimental Evaluation of Two-Phase Heat Transfer Coefficient Under Oscillatory Flow Conditions and Correlation Development

Abstract Natural circulation boiling water reactors (NCBWRs) are prone to flow instabilities. Due to instabilities, heat transfer deteriorates. However, the extent to which the heat transfer coefficient (HTC) deteriorates is not well established. In this work, experiments are conducted under controlled flow oscillations to evaluate the impact of the time period, mean mass flux, and amplitude ratio on HTC. Existing correlations for HTC are compared with experimental results and are found to be not satisfactory. To improve the predictions of HTC, correction factors (CFs) are proposed for existing correlations. Further, an improved HTC correlation has been developed, incorporating new nondimensional numbers.

Effect of gradually expanding flow passages on flow boiling of micro pin fin heat sinks

In this paper, effect of gradually expanding flow passages on saturated flow boiling characteristics of micro pin fin heat sinks is experimentally investigated. In this regard, a comparative analysis is performed based on three different heat sinks having straight parallel channels, uniformly distributed micro pin fins, and decreasing number of micro pin fins in flow direction. For these three geometries, visualization supported experiments are conducted, approximately, at the mass flux of 212 kg m–2 s–1, inlet temperature of 75 ± 1 ᵒC and effective heat fluxes of 132 – 272 kW m–2. Working fluid was deionized water. It is concluded that gradually expanding flow passages towards the outlet significantly improves the two-phase heat transfer coefficient up to nearly 501% and 365% compared to straight parallel channels and uniformly distributed micro pin fins, respectively. The structure having decreasing number of micro pin fins obviously suppresses flow boiling instabilities, and also presents lower pressure drop (lower up to 1.45 times) than uniformly distributed micro pin fins. Gradually enlarging flow passages provide escape-ways in the highly saturated region; thus, accelerate the flow and shorten the waiting period of vapor on heat transfer surface. Important conclusions are obtained regarding flow mechanism via flow images.

An optimization approach in the development of a new correlation for two-phase heat transfer coefficient of R744 in a microchannel

To date, no single method has been found to satisfactorily predict the two-phase heat transfer coefficient of R744 refrigerant in small channels. Studies are continuously being done to obtain a coefficient with acceptable mean absolute error (MAE) which measures the difference between the predicted and experimentally determined coefficient values. It is important to have available accurate heat transfer coefficient correlation for the two-phase heat transfer coefficient so that a compact heat exchanger that maximizes device performance while reducing cost and energy needs can be designed. In this study, genetic algorithm (GA) is used as an optimization tool to achieve a more accurate correlation for R744 in a microchannel by minimizing the MAE. Over 536 sets of experimental data from previous studies were utilized, optimizing the six constants appearing in the force convective factor, F, and nucleate boiling suppression factor, S, of the selected superposition correlation. The results showed that the MAE between the newly optimized correlation and selected experimental data at all ranges of vapor quality has been successfully reduced from 38.39 to 34.40%. With more available data, the suggested method can be utilized to achieve a more accurate empirical prediction that matches well with the experimental data.