Two-phase Flow Boiling Heat Transfer Research Articles

Evaluation of heat transfer correlations for two-phase flow boiling in twisted tapes inserted tubes

Many experimental researches were conducted in the twisted tapes inserted tubes, and many correlations were proposed and evaluated. However, most of the evaluations were based on specific experimental data, and the number of correlations evaluated was limited in each paper. This paper conducted a review of correlations and experimental investigations of the two-phase flow boiling heat transfer coefficient for twisted tapes inserted tubes, the key forms of the 5 correlations were reviewed, and the prediction accuracy of the 5 correlations was evaluated against the 508 published experimental data. This paper proposed a new correlation which has a MAD of 17.2%, 10.2% lower than the best existing correlation.

Thermal Characterization of Subcooled Flow Boiling in a Pin-Fin Coldplate With Non-Uniform Heating

Abstract Coldplates are a crucial component in various cooling applications, such as cooling data center servers and power electronics. The unprecedented growth in electronics power density, along with the resulting ultrahigh heat fluxes, demands a transition from single-phase forced convection to two-phase flow boiling heat transfer. The majority of studies in the literature have focused on flow boiling in fin-enhanced silicon microgaps and microchannels, with only a few addressing flow boiling in millimeter-scale heat sinks. In the present study, flow boiling of HFE-7200 dielectric fluid in a millimeter-scale pin-fin coldplate is experimentally investigated under nonuniform heating conditions. Four background heaters represent the low-dissipating-power devices. On the other hand, five hotspot heaters mimic the high-heat-flux devices and generate heat fluxes ranging from 50 W/cm2 to 1000 W/cm2, corresponding to hotspot heat inputs ranging from 62.5 W to 1.25 kW, respectively. The coldplate's thermohydraulic performance is investigated for various flow rates and inlet temperature ranging from 0.5 L/min to 1.5 L/min and from 25 °C to 60 °C, respectively. A high-speed camera is utilized for a narrow field of view (FOV) flow visualization at a frame rate of 2229 fps while a digital camera is used for a wider FOV at 60 fps. Flow visualization demonstrated the transition between bubbly, slug/churn, and stratified two-phase flow regimes.

Numerical simulation of boiling behavior in vertical microchannels

High heat flux electronic devices put forward new requirements for heat dissipation, and boiling heat transfer technology is widely used because of its higher heat dissipation capacity. In this study, the volume of fluid method was employed, along with the incorporation of the Lee phase-change mass transfer model, to investigate two-phase flow and heat transfer in vertical upward rectangular microchannels. The heat flux was varied within the range of 10–40 kW/m2, while the mass flux was varied within the range of 200–600 kg/m2 s. With the increase in heat flux, bubble flow, slug flow, churn flow, and annular flow were found successively. A phase diagram was established to predict the flow pattern transition during the boiling process. When the flow pattern changes to the churn and the annular flow, the active nucleation site density increases obviously with the Boiling number (Bo). A new correlation was proposed for two-phase flow boiling heat transfer, suitable for vertical upward channels in microscale fluids. The friction factor obtained using the Darcy friction factor equation agrees well with the simulation results at a high-pressure drop. The instability in microchannels increases with the increase in heat flux, particularly in annular flow, resulting in more severe wall temperature fluctuations.

Saturated flow boiling heat transfer of R1233zd(E) in parallel mini-channels: Experimental study and flow-pattern-based prediction

The parallel channel is widely used in the two-phase flow boiling heat transfer due to its simple structure, but the existence of multiple flow patterns causes a complex and non-unified explanation of the heat transfer mechanisms. In this study, a saturated flow boiling experiment was designed in a parallel mini channel heat sink with R1233zd(E) as the working fluid. By controlling the vapor quality difference at the inlet and outlet to around 0.1–0.2, each single flow pattern was isolated and presented in the test section, and the corresponding variations of heat transfer coefficient (HTC) were discussed. The results showed that the flow pattern during the saturated flow boiling included bubbly flow, slug flow, wavy-annular flow and local dry-out. The transition boundaries between bubbly flow, slug flow and wavy-annular flow showed good agreement with the present criteria, while a modification was made on the transition from wavy-annular flow to local dry-out. The HTC showed a fast increase under low vapor qualities, then became stable and decreased with the increase of vapor quality. In addition, 6 correlations were selected to compare with the measured HTC classified according to the flow patterns. The comparison showed that the present correlations had different prediction accuracies on the HTC data under each flow pattern. The best prediction on the heat transfer during bubbly and slug flow was achieved by Liu and Winterton's model with the MAE of 16.4% and 12.9%, while Li and Wu's model showed the best agreement with the HTC data during wavy-annular flow with the MAE of 9.1%.

A Comparative Analysis of Two-Phase Flow Boiling Heat Transfer Coefficient and Correlations for Hydrocarbons and Ethanol

This study will present a comprehensive review of the two-phase flow boiling heat transfer coefficient of hydrocarbons such as propane (R-290), butane (R-600), iso-butane (R-600a), and ethanol at various experimental conditions. Studying the multiphase flow heat transfer coefficient is crucial for many types of heat transfer equipment to achieve higher efficiency for more compact design and cost reduction. One reason we chose hydrocarbons as refrigerants in this study is that they are of an ozone depletion potential equal to zero (ODP = 0) and a deficient level of direct global warming potential (GWP = 3). Moreover, hydrocarbons’ thermodynamic and thermophysical characteristics qualify them to be a strong candidate for more heat transfer applications, initially, by constructing a database for the working fluids using multiple existing experimental work. The current data that this study have collected for the flow boiling spans a wide range of parameters, such as mass flux, heat flux, operating pressure, saturation temperature, etc. Furthermore, by comparing the experimental multiphase heat transfer coefficient database with the anticipated values of each correlation, the prediction performance of 26 correlations found in the literature was assessed. This study allows the best prediction method to be selected based on the minimum deviation of predicted results from the experimental database provided based on the mean absolute error (MAE) calculated from the assessed correlations. The conclusions of such a study can also be helpful for developing more accurate correlation methods for these fluids and improving the prediction of their flow boiling characteristics.

Aspects of Two-Phase Flow Boiling Heat Transfer inside Tube of Water Tube Boiler—A Numerical Study

Aspects of Two-Phase Flow Boiling Heat Transfer inside Tube of Water Tube Boiler—A Numerical Study

A Comprehensive Assessment of Two-Phase Flow Boiling Heat Transfer in Micro-Fin Tubes Using Pure and Blended Eco-Friendly Refrigerants

This review study examines flow boiling heat transfer in micro-fin tubes using mixed and pure refrigerants with zero ozone-depleting potential (ODP) and minimal global warming potential (GWP). This investigation focuses on the extraordinary relationship between heat transfer coefficients (HTCs) and vapor quality. Since the introduction of micro-fin heat exchanger tubes over 30 years ago, refrigerant-based cooling has improved significantly. Air conditioning and refrigeration companies are replacing widely used refrigerants, with substantial global warming impacts. When space, weight, or efficiency are limited, micro-fin heat exchangers with improved dependability are preferred. This review article discusses flow boiling concepts. The researchers used several refrigerants under different testing conditions and with varying micro-fin tube parameters. Micro-fin tubes are promising for improved heat transfer techniques. This tube increases the heat transfer area, fluid disturbance, flow speed, and direction owing to centrifugal force and HTC. As the focus shifts to improving heat transfer, pressure drop, mean vapor quality, and practical devices, this subject will grow more intriguing. A radical shift will reduce equipment size for certain traditional heat transfer systems and bring new products using micro-scale technologies. This suggested review effort helps comprehend saturation flow boiling through micro-fin tubes and find the right correlation for a given application. This domain’s challenges and future relevance are also discussed.

Investigation on the two-phase flow and heat transfer behaviors in a new central uniform dispersion-type heat exchanger

In order to improve the temperature uniformity of multiple chips, a new central uniform distributed heat exchanger (CUD) is designed. Firstly, adopting ethanol solution as coolant, the cooling ability of the heat exchanger is numerically investigated by using VOF gas-liquid two-phase flow model, and the two-phase flow experimental system is established to verify the correctness of the numerical results. The results show that the Tavg (average temperature of heat sources) error range between the experimental and numerical values is 1 K ~ 3 K, and the relative error of SDT (standard deviation of temperature) is 6% ~ 10%. This verifies the accuracy of the numerical simulation method. Secondly, the two-phase flow boiling heat transfer behavior are compared with that of single-phase flow with water as coolant. The results show that with the increase of inlet flow, the thermal resistance and average temperature of two-phase flow change are less than that of single-phase flow. This indicating that two-phase flow has better advantages in improving the temperature uniformity of multiple heat sources. Thirdly, the heat transfer characteristics of radiators with different Wi are studied. Compared with Case1, the boiling heat transfer coefficient of Case3 and Case4 can be increased by 12.3% and 18.3%, respectively. However, Case3 is a better choice by weighing the pressure drop. Finally, the prediction functions of the boiling heat transfer coefficient and pressure drop is established, and the multi-objective genetic algorithm is used to obtain the best parameters of this heat exchanger. The average heat transfer coefficient of the optimized heat exchanger can be increased by 7.56%, and the pressure drop can be reduced by 3.54% compared to the exchanger before optimization. Furthermore, the heat dissipation ability of the optimized heat exchanger design is studied numerically. The simulated results are in good agreement with the optimization results.

Experimental Investigation of Two-Phase Flow Boiling Heat Transfer Coefficient and Pressure Drop of R448A inside Multiport Mini-Channel Tube

Regulations and restrictions against high global warming potential (GWP) refrigerants have been introduced to encourage the adoption of environmentally friendly refrigerants and mitigate the environmental impact of the HVAC industry. R448A, a zeotropic blend with a GWP of 1390, has recently been proposed as a drop-in replacement for R404A and R410A in commercial systems. In this study, the heat transfer coefficient and pressure drop characteristics of R448A within a multiport mini-channel tube were experimentally investigated. The experimental ranges of the mass and heat fluxes were 100 to 500 kg/(m2s) and 3–15 kW/m2, respectively. Additionally, the range of quality from 0 to 1 was considered at two fixed saturated temperatures of 3 and 6 °C. The heat transfer coefficient increased with mass flux. Under low mass flux condition, the heat flux increased the heat transfer coefficient, but there was no noticeable effect of the saturated temperature on the heat transfer coefficient. At high mass flux, heat flux had no major effect on heat transfer, while a decrease in the saturated temperature was found to increase the heat transfer coefficient. Moreover, the pressure drop increased with an increase in the mass flux and vapor quality, whereas the heat flux did not affect the pressure drop. The heat transfer coefficient and pressure drop performance of R448A was compared with that of R410A inside the same tube. Finally, correlations for heat transfer coefficient and pressure drop were proposed for the prediction of heat transfer coefficient and pressure drop in practical applications.

Numerical simulation research on two-phase flow boiling heat transfer in helically coiled tube

In this paper, the partial geometric models of helically coiled tube type heat exchanger with different tube diameters (d = 12, 19, 25 mm), tube pitches (P = 50, 100, 150 mm) and helical radius (R = 100, 150, 200 mm), were established. Numerical simulation and performance comparison were carried out using water and water vapor as the working fluid. The gas–liquid phase distribution in the helically coiled tube, the gas volume fraction at different cross-sections, and the changes of heat transfer coefficient and pressure drop along the length of the helically coiled tube were analyzed. The results showed that the gas phase was concentrated in the inner side (near the side of the central axis) and the upper side of the helically coiled tube under boiling condition, and the liquid phase was concentrated in the bottom of the helically coiled tube and is stratified with the gas phase. As the helical radius and tube diameters increased, the heat transfer coefficient and unit pressure drop decreased gradually. Based on the simulated data, the boiling heat transfer correlation was fitted by the method of multiple nonlinear regression, and the mean absolute error was 4.5% compared with the modified Chen's correlation. Therefore, the numerical results were correct and trustable.

Simulational Research on Two-Phase Flow Boiling Heat Transfer in Helically Coiled Tube

Simulational Research on Two-Phase Flow Boiling Heat Transfer in Helically Coiled Tube

Boiling Heat Transfer and Visualization for R717 in a Horizontal Smooth Mini-tube

The paper presents an experimental and theoretical investigation on the two-phase flow boiling heat transfer with ammonia (R717) as the refrigerant in a horizontal smooth mini-tube with inner diameter of 3mm. The experimental data were obtained in the following condition, the heat flux density is 10∼30 kW · m − 2 , the mass flux is 40∼200 kg · m − 2 · s − 1 , the saturation temperature is -10∼10 ℃, and the range of vapor quality is 0.1 ∼1. The experimental results show that the heat transfer coefficient before dryout phenomenon increases and gradually reaches a peak with the increase of vapor quality, but occurs heat transfer deterioration after dryout phenomenon. Combined with the transform of flow patterns, the effect of mass flux, heat flux density, and saturation temperature on the heat transfer coefficient and friction pressure drop are analyzed. The theoretical gas phase flow velocity of ammonia is much higher than other refrigerants, and the flow boiling heat transfer for ammonia is more dependent on convective boiling. The existing correlations of two-phase flow are compared with the measured data of ammonia. According to the experimental and comparative results, Kew and Conwell correlation and Müller-Steinhagen and Heck correlation were updated respectively, and new boiling heat transfer correlation and frictional pressure drop correlation were obtained.

Novel Saturated Flow Boiling Correlations for R600a and R717 Refrigerants

This research study provides a detailed analysis of the two-phase flow boiling heat transfer coefficients of R600a and R717. In addition, it proposes novel saturated flow boiling heat transfer correlations for these two refrigerants. In this context, 1306 experimental data points for R600a and 885 data samples for R717 are extracted from the literature works to develop new flow boiling models. Two different correlations are constructed under different forms of baseline frameworks for each refrigerant. The proposed flow boiling model for R600a utilizes the dimensionless numbers and operational parameters of the best performing literature correlations for this refrigerant while the model developed for R717 takes the advantage of piecewise continuous correlation form which successfully trace the tendencies of heat transfer coefficient with varying vapor qualities. It is found that the new model for R600a has a mean absolute error of 12.4% and mean relative error of 0.1, predicting 78.5% of the entire database within ±20.0% error band whereas the proposed correlation for R717 has an absolute error value of 17.3% having 65.3% of the data within ±20.0% error band, which are much better and accurate estimations compared to those obtained by the existing flow boiling models for R717 refrigerant.

Effect of bubble coalescence on two-phase flow boiling heat transfer in raccoon microchannel - A numerical study

A three-dimensional numerical study on two-phase flow boiling is conducted for raccoon and straight microchannels. The effect of flow boiling instability is observed under conjugate conditions with water as coolant. Bubble coalescence in bubble dynamics is carefully studied, and the forces acting are identified. Eotvos and Weber number are calculated to determine the bubble behavior and shape in a raccoon microchannel. Boiling number is also considered to predict the two-phase flow boiling heat transfer. The bubble flow pattern for two-phase boiling in straight and raccoon microchannel are investigated under different parameters such as Waviness (γ∼0 - 0.267) and Mass flux (G ∼ 118 – 590 kg/m2s), keeping saturation temperature constant at Tsat = 310 K corresponding to pressure (p sat = 6500 pa). It is observed that the Eotvos and Boiling numbers are comparatively higher for values of waviness 0 – 0.067 than other raccoon microchannels. This happens due to the presence of long elongated bubbles, causing the flow boiling instability. On the other hand, as the waviness increases, the bubble size changes periodically. Effect of forces on bubble coalescence for different waviness and mass flux is also observed.

Experimental study on saturated flow boiling heat transfer of R1234yf in a horizontal 2.01 mm tube under hypergravity

Experiments on R1234yf flow boiling in a horizontal 2.01 mm copper tube under hypergravity are conducted to investigate the effect of gravity on two-phase flow boiling heat transfer coefficient (HTC). The experimental range of parameters is gravity levels of 1–3.16 g, mass fluxes of 365 and 570 kg m−2 s−1, saturation pressures of 0.62 and 0.70 MPa, heat fluxes of 47.6 and 97.5 kW m−2, and vapor qualities up to 0.985. Based on the experimental data, the effects of gravity, quality, heat flux, mass flux, and saturation pressure on the HTCs were analyzed. The results show that the HTCs increase with increasing gravity levels for the qualities up to around 0.7, and the gravity effects become indistinct and inconsistent when the quality is high (x > 0.8). The trends of the HTC variation with quality and the effects of the heat flux, mass flux and saturation pressure on the HTCs are similar under different gravity levels. When x < 0.6, the nucleate flow boiling mechanism dominates with the HTCs varying little with quality, and the HTCs increase significantly with increasing heat flux and negligibly with increasing mass flux and pressure. The critical heat fluxes occur when x > 0.7 or higher, and they occur earlier when the gravity level is higher. The comparison of the experimental data with existing saturated flow boiling HTCs for normal gravity shows that the Fang (2013b) correlation for R134a performs best with a mean absolute deviation (MAD), and that all of the top four correlations in prediction accuracy under-predict the experimental HTCs, which is consistent with the experimental results that hypergravity enhances flow boiling heat transfer.

Investigation on the temperature distribution in the two-phase spider netted microchannel network heat sink with non-uniform heat flux

The considerable increase of the heat flux and chips density on the phased array antenna has brought great challenges to the thermal management. The objective of the work is to explore an effective scheme to achieve ideal uniform temperature distribution and lower maximum temperature with the condition of non-uniform heat flux. Taking into account of the high efficiency of microchannel heat sink and the promising performance of the two-phase flow boiling heat transfer due to its merit in keeping uniform surface temperature, a novel two-phase spider netted microchannel network heat sink is proposed in this paper. Firstly, a three-dimensional fluid-solid-thermal conjugate model is developed to investigate the flow boiling heat transfer performance of the spider-netted microchannel network numerically. During the numerical calculation, a multi-scale meshing generation method is proposed to reduce modeling difficulties on the premise of proper mesh quality. Then, the flow boiling heat transfer experiment is conducted to verify the reasonable of the model and the simulation results. Finally, the flow boiling heat transfer performance in the spider netted microchannel network is compared to the typical straight microchannel network. Results show that the proposed structure yields better flow boiling heat transfer performance with lower thermal resistance and more satisfactory temperature uniformity mainly due to its uniform flow distribution and enhanced vapor movement. It is also found that the temperature uniformity has reached 1.58 °C at heat flux of 100 W/cm2, but it decreases with the increase of heat flux. This situation cannot be effectively solved by means of increasing inlet flow rate. Thus the structure optimization is conducted to enhance the flow boiling heat transfer performance further, and the optimal structure can meet requirements of temperature uniformity within 2 °C at higher heat flux of 150 W/cm2.

Experimental Characterization of Two-Phase Cold Plates Intended for High-Density Data Center Servers Using a Dielectric Fluid

Abstract High-performance computing (HPC) data centers demand cutting edge cooling techniques like direct contact liquid cooling (DCLC) for safe and secure operation of their high-power density servers. The two-phase flow boiling heat transfer technique is widely believed to address the heating problem posed by HPC racks. In this study, a novel liquid-cooled cold plate containing microchannel and jet impingement arrangement was characterized for its two-phase flow and thermal behavior. A sophisticated bench top setup involving a mock package was developed to carry out the experiments in a controlled fashion using a dielectric fluid Novec/HFE-7000. Two-phase flow boiling in cold plates which has a strong dependency on surface phenomena were carefully studied at various levels of inlet pressure, subcooling, flow rates, and heat flux levels to the mock package. A resistance network was invoked to determine the average heat transfer coefficient at various exit qualities estimated by the energy balance equation. While the results make it evident that, the high heat generating components can be kept at operable conditions using the two-phase cooling; a deeper insight at the outcomes could pave way for more energy efficient cold plate designs. The experiment was carried out with a large heated surface of 6.45 cm2 and maximum dissipated heat flux was around 63.6 W/cm2 corresponding to chip power of 410 W. Base temperature was kept below 75 °C and pressure drop did not exceed 21 kPa.

Gradient wick channels for enhanced flow boiling HTC and delayed CHF

Liquid supply to wick structures of flow boiling surfaces is fundamentally restricted by the capillary limit at which the pressure drop of the wicked liquid surpasses texture-amplified capillary forces. Gradient wick structures partially decouple permeability and capillary pressure, thereby delaying the capillary limit. In this study, gradient wick channels facilitating out-of-plane liquid delivery are introduced to postpone the capillary limit and thus enhance the two-phase flow boiling heat transfer coefficient (HTC) and delay critical heat flux (CHF). Here, the permeability of the gradient wick channels is augmented by large-pore-size meshes employed near the bulk fluid while capillary pressure is maximized by small-pore-size meshes utilized near the hot boiling surface. This combination of wick structures enables to preferentially guide the cooling liquid, deionized water, from the far-field cold liquid toward the bottom hot substrate. The spatial distribution of individual gradient wick channels promotes separate liquid-vapor pathways, thus facilitating the vapor escape process. Experiments conducted here reveal that the flow boiling performance metrics of the proposed heat sink leveraging the gradient wick channels outperform those of the homogenous wick channels and solid fin channels. The proposed heat sink demonstrates a strong liquid mass flux dependency due to a combination of convective boiling and amplified wickability effects. The enhanced convective boiling could be related to surface roughness, a high number of active nucleation sites, and a large surface area available through tortuous passages of wick channels. At higher mass flow rates, effective capillary pressure available for out-of-plane wicking action also increases, thereby further boosting the wickability effect and associated heat transfer processes. This could meaningfully delay the CHF. In fact, the CHF limit was not observed on the gradient wick channel surfaces in the mass flux and wall superheat range studied. The experimental results indicated a maximum heat flux of 870 W/cm2 with a gradient wick channel heat sink, a 60% improvement compared with a plain copper surface. Furthermore, a maximum HTC of 1000 kW/m2-K at a wall superheat of 3 °C was observed, a three-fold enhancement compared with that of the plain surface. The proposed gradient wick channel topology offers new pathways for designing innovative surface technologies with high heat removal capabilities, thereby potentially improving the energy economy in myriad modern energy applications.

Two-phase flow boiling in a microfluidic channel at high mass flux

We report the experimental investigations of two-phase flow boiling heat transfer characteristics of a refrigerant in a microfluidic channel at a high mass flux (more than 1000 kg/m2 s). We investigate the heat transfer coefficients at a heat flux range of 7.63 kW/m2–49.46 kW/m2, mass flux range of 600 kg/m2 s–1400 kg/m2 s (high mass flux), and saturation temperature range of 23 °C–31 °C. We propose the new two-phase flow boiling heat transfer correlation of a refrigerant, which is used as the working fluid for the present experiments, at the microfluidic scale. We experimentally establish the functional relationship of two-phase flow boiling heat transfer correlation of the refrigerant during flow boiling in a rectangular microchannel with the Reynolds number, the boiling number, and the Weber number. We believe that the inferences of this study may provide a design basis for the micro-heat exchanger, typically used for thermal management in electronic devices, micro-electro-mechanical systems, and electric vehicle battery cooling system.

Experimental investigation of two phase flow boiling heat transfer of mixtures of refrigerant R410A and nanolubricants in a horizontal smooth copper tube

In air conditioning systems, a small amount of lubricant leaves the compressor and circulates through the system. The lubricant acts as a contaminant, affecting heat transfer and pressure losses in heat exchangers. The mixture of refrigerant and nanolubricants (NLs), that is, nanoparticles dispersed in the lubricant oil, have shown potential to augment the two-phase flow boiling heat transfer in the literature. However, the mechanisms of heat transfer enhancements due to the NLs are still not well understood. Published models of two-phase flow boiling lack experimental validation with refrigerant and NL mixture. This work addresses this gap by presenting new experimental heat transfer coefficient and pressure drop results for the saturated two-phase flow boiling of R410A with two NLs in a 9.5 mm I.D. smooth copper tube: a ZnO nanoparticle laden lubricant and a γ-Al2O3 nanoparticle laden lubricant. In smooth tube, R410A–NL mixtures, which had higher thermal conductivity and kinematic viscosity than R410A–POE lubricant mixture, showed degradation in heat transfer coefficient compared to R410A–POE lubricant case; but they also had lower pressure drops. Relocation of the nanoparticles closer to the inner wall of the tube could lead to enhancement in heat transfer at an expense of increased frictional pressure drop.