Evaporation Heat Transfer Characteristics Research Articles

Research on Heat Transfer Characteristics of Plate Evaporators for Cold Storage

Research on Heat Transfer Characteristics of Plate Evaporators for Cold Storage

Wetting dynamics and heat transfer of droplet evaporation around boiling point: Facile manipulation by surface roughness

The evaporation of water–ethanol droplets on solid surfaces has vast potential for applications in thermal management, microfluidics, and biomedical sciences, while the approach of manipulating wetting dynamics and heat transfer of droplet evaporation around the boiling point remains open for investigations. The present study proposes a facile approach: fabricating solid surfaces with diverse roughness by sandpaper milling with different grit densities, which can alter the wetting and heat transfer characteristics of droplet evaporation. We discover that when the temperature of the heated surface exceeds the droplet boiling point, the existence of bubbles affects the solid–liquid wetting state; surface roughness alters the ability of bubble departure, thus determining the solid–liquid contact and the contact line retraction time. Consequently, in this case, the solid–liquid contact area and the heat absorbed from heated surfaces for droplets changes with surface roughness, altering the droplet evaporation time and the average heat flux at the solid–liquid interface. Bridged by droplet wetting dynamics, the heat transfer of droplet evaporation above the boiling point can be manipulated by adjusting surface roughness, offering a facile approach to tuning the process of droplet evaporation in the related industrial applications.

Thermal performance of a two-pass microchannel evaporator

Microchannel evaporator is one of the key components in many thermal systems such as heat exchanger of HAVC and refrigeration system. The evaporation heat transfer performance of microchannel evaporator, which usually has many flow paths, is highly affected by the distribution of liquid flow that will seriously deteriorate the flow boiling performance due to the lack of liquid supply, local dryout, etc. In this study, a novel two-pass microchannel evaporator (L × W × H = 103 × 64 × 20 mm) is designed and fabricated. The width, height and length of each individual channel are 0.6, 1 and 65 mm, respectively. To significantly improve liquid redistribution after the first-pass, evenly distributed spacers with different diameter holes were placed in the connecting flow path. Additionally, the channel number ratio between the first and second pass plays an important role. Therefore, three different channel ratios of 1:1, 1:2, and 3:5 were selected in this study. The heat transfer and flow characteristics of the two-pass microchannel evaporator using working fluid HFE-7100 were experimentally studied for flow rate varying from 30 ml/min to 110 ml/min. The inlet subcooling keeps about 30 K. Also, visualization studies were conducted for the explanations. The experimental results indicate that the thermal performance is superior in microchannel evaporator with channel ratio of 1:2. By considering Reynolds number of each channel in the first pass, microchannel evaporator with channel ratio of 1:1 outperforms other two evaporators. Furthermore, the wall temperature uniformity of this evaporator is better than others. The achieved heat flux of 20 W/cm2 is much higher than the reported studies. The overall two-phase HTC of this microchannel evaporator is about 4.5 kW/m2K.

Visualization experimental study on evaporation heat transfer characteristics of an open evaporator with a tower-like porous wick

Temperature overshoot often occurs in loop heat pipe, which is caused by the large bubble in the compensation chamber. But it is still uncertain whether this large bubble grew from parasitic bubbles inside porous wick. A multi-layer tower-like porous wick is designed to understand the growth and expansion of parasitic bubbles. The parasitic bubbles are induced to occur inside porous wick by inserting a copper pillar on the evaporator wall, and the multi-layer porous wick is used to eliminate parasitic bubbles. The pumping stagnation occurs under certain heat loads, but temperature overshoot of the evaporator wall no longer occurs in the open evaporator with the tower-like porous wick, which guides the design of evaporator to avoid temperature overshoot. It is found that a lot of small bubbles only appears on the lower surface of porous wick immersed in liquid in the evaporator, while not appears on the upper surface. Three groups of repeated experiments are carried out sequentially and the performance of the evaporation heat transfer gradually deteriorates, which is speculated to be caused by a gap between porous wick and the copper pillar, larger pore size next to copper pillar, or poor wetting caused by copper pillar oxidation. Two different types of the evaporator overheating are discovered, including the overall retreat of the evaporating interface and dry-out region expanded from the internal of porous wick to the external. At high heat load, the evaporating interface expands along the copper pillar, causing an increase in the heat transfer coefficient of the evaporator. The research results indicate that in order to better utilize porous wick for evaporation heat transfer, it is necessary to reliably adhere porous wick to the heating surface and avoid the formation of the large bubble in the compensation chamber.

A mesoscopic approach for nanoscale evaporation heat transfer characteristics

We propose a mesoscopic approach for investigating steady/transient nanoscale evaporation heat transfer characteristics, whose kinetic nature makes it an ideal tool to model the evaporation kinetics at the liquid-vapor interface and the microscopic wall-fluid interactions. Simulation of the evaporation of a flat nanoscale thin liquid film demonstrates its capability of resolving kinetically limited evaporation and liquid film adsorption. For the simulation of an evaporating meniscus in a nanochannel, our proposed mesoscopic approach considers the conservation of mass, momentum and energy of both liquid and vapor phases, thereby naturally incorporating both the multi-dimensional heat transfer effect and vapor transport resistance in nano-confined space. We demonstrate that solid-fluid interaction plays dominant roles on the interfacial transport during nanoscale evaporation, and vapor transport resistance has nonnegligible influences on the evaporation heat/mass transport in nano-confined spaces. By handling statistical behavior of molecule ensembles, this mesoscopic approach not only preserves microscopic physics but also has high computational efficiencies, enabling the simulation of space and time domains on the practical application level. This work paves the way for quantitative investigations of steady/transient nanoscale evaporation heat transfer characteristics using mesoscopic approach.

CFD Air Flow Evaluation of Finned Tube Evaporator for Refrigerated Display Cabinet Application

This study is aimed to develop a simulation to improve the performance of the finned tube evaporator which is applied to the refrigerated display cabinet. CFD model was developed to be able to analyse the characteristics of air flow inside the fin gap and air side heat transfer coefficient. Geometry of the model of overall finned tube evaporator is considered covering two aluminium wavy fins with an air flow in between, combination of staggered cooper tubes with refrigerant flow inside. Fin gap is designed 4 mm to anticipate frost on the fin surface that can block air flow. Turbulence models used in the study is the realizable k-ε turbulence which had the best performance turbulence model and it was validated with secondary data from previous studies and shows the lowest error only 5.9 %. The use of CFD was found to be sufficiently representative of the heat transfer characteristics of evaporators, and acted as an effective simulation tool to determine the heat transfer coefficient in order to improve efficiency in terms of improved design. The characteristics of air flow between the fin gap and around the tube was obtained various and complex. In the case study the entry velocity of 1.7 m /s at the highest turbulence condition of the first row can reach speeds of 2.75 m/s. Hight turbulence regime in flow can indicate higher the heat transfer coefficient of the evaporator.

Investigation on performance enhancement of micro-channel separated heat pipe in data center: A coupled heat-mass-flow characterization approach

As data center continues to expand in scale and complexity, the demand for efficient heat dissipation solutions becomes increasingly critical. The utilization of micro-channel separated heat pipe is prevalent in data center cooling due to their low energy consumption and efficient cooling. This study aims to investigate the effects of different refrigerant flow rates, different inclination angles and different inlet vapor phase fractions on the heat transfer and flow characteristics of micro-channel separated heat pipe evaporator. Firstly, a simulation model of the heat pipe evaporator was established considering the heat-mass-flow coupling characteristics, and the fins were modeled and simplified without neglecting the enhanced heat transfer. Then, the experimental data are used to verify the established simulation model, and the error is within the acceptable range. Finally, scheme design and performance optimization research are conducted. Research on the inlet flow rate of R134a shows that when the inlet flow rate is 0.005 m/s, the outlet is superheated vapor. The presence of an inclination angle (0°-20°) in the evaporator not only significantly reduces the pressure difference between the outlet and the inlet (more than 30 %), but also improves the heat transfer coefficient and the refrigerant velocity magnitude. In addition, increasing the inlet vapor volume fraction promotes vapor bubble nucleation and reduces the pressure drop, but leads to the presence of outlet overheating. The findings of this study contribute valuable insights for advancing thermal management strategies in data center environments.

Machine learning-based Nusselt number prediction for falling-film evaporators in absorption refrigeration systems

The evaporator serves as a pivotal component of an absorption refrigeration system (ARS), bolstered by improved wettability and mitigation of the irregular arrangement of the liquid film. The complexities of the evaporation heat transfer characteristics of falling films have led to numerous empirical correlations for falling-film Nusselt number (Nu) estimation, demanding substantial computational resources and experimental expenses and limiting their universal applicability. This study used machine learning to predict Nu of a falling-film evaporator. Artificial neural network (ANN) and random forest regression (RFR) models were used to predict heat transfer characteristics and establish a novel Nu correlation using an experimentally derived dataset. Correlations of falling-film Nu were established for six tube types using empirical data, ANN, and RFR predictions and then compared based on experimental falling-film Nu. The ANN and RFR provided high prediction performance within 10 % error and an R2 of 0.985 and 0.999, respectively. The Nu correlations derived from ANN and RFR demonstrated satisfactory agreement, typically within ±15 % error of experimental results lower than that of the empirical correlations. The optimized ANN and RFR demonstrates their promising ability to predict heat transfer characteristics and establish Nu correlations, thereby reducing the need for immense experimental efforts.

Evaporation performance of propane in offset strip-fin plate heat exchanger for refrigerant-based battery thermal management in electric vehicles

This study investigates the potential of refrigerant-based cooling (RC) using a natural refrigerant (propane) and lightweight energy-efficient brazed plate heat exchangers (PHXs) in the battery thermal management systems (BTMSs) of the electric vehicles (EVs). Experimental assessments were conducted to analyze the evaporative heat transfer characteristics of propane within an offset strip-fin flow-structured PHX under various operational conditions resembling the BTMS operation. The battery discharge rates, often represented by the C-rate, are determined by the discharging current relative to the battery's capacity. In this study, a heat generation model based on the C-rate was employed to evaluate the thermal performance of the battery. Parameters, such as mass flux (G) at 30–70 kg/m2s, discharge rates (C-rate) of 1, 1.25, and 1.5C, and saturation temperatures (Tsat) of 16–24 °C were explored, with the inlet vapor quality (xin) in the range of 0.1–0.8. The influence of G and Tsat on the heat transfer performance showed that higher G values (60 and 70 kg/m2s) achieved the maximum heat transfer coefficients (HTC), which resulted in impractical pressure drops (PD). While higher HTC and lower PD were observed at a Tsat of 24 °C, concerns about increased inner wall temperature (Tiw) and safety emerged at this temperature. Hence, the optimum conditions of G (30 and 50 kg/m2s) and Tsat (16 and 20 °C) were investigated. The average HTC increased by 29, 27, and 21 % at 50 kg/m2s compared to that at 30 kg/m2s with Tsat of 20 °C, resulting in an average of 0.7, 0.9, and 1.2 °C reduction in the Tiw for C-rates of 1.5, 1.25, and 1C respectively. Compared to the Tsat of 16 °C, higher HTC and lower PD were observed at 20 °C, however, the Tiw was in the range of 21.12–20.47 °C for Tsat of 20 °C, which is 3.5–4 °C higher than that at Tsat of 16 °C. The results from the critical heat transfer coefficient analysis showed that the peak heat dissipation with minimal wall temperature was attained with optimal xm of 0.45–0.5, which enhanced the cooling efficiency for both the G values of 30 and 50 kg/m2s, and both the Tsat values (16 and 20 °C).

Experimental study on evaporation heat transfer characteristics of multi-composite porous wick with three different powders

Based on our previous research, the composite porous wick with spherical-dendritic powders owns biporous structure and uneven local thermal conductivity. In this paper, a multi-composite porous wick sintered with three different materials and structures of spherical copper, spherical nickel and dendritic nickel is tested. The spherical copper is beneficial to expansion of the evaporation heat transfer area, the dendritic nickel can provide sufficient liquid for evaporation, and the spherical nickel is used to regulate the distribution of spherical-dendritic powders and large pores. Through the evaporation heat transfer experiment, it is found that there are five different types of operating temperature and evaporation mass curves, including continuous and intermittent overshoot, single overshoot, stable operation and temperature losing control. The occurrence of the overshoot is affected by the bubble behavior in the bottom of the porous wick. And two types of the vapor–liquid interface behavior occur in the overshoot operation, in which the porous wick is penetrated by a single bubble or the evaporating interface, respectively. The multi-composite porous wick can reach a maximum heat load of 520 W (the corresponding heat flux is 43.5 W/cm2), and the highest heat transfer coefficient of the open capillary evaporator can reach 2776.1 W/(m2·K), with the lowest total thermal resistance is 0.3089 K/W at 500 W heat load. As a reference using a composite porous wick prepared with spherical copper powder and dendritic nickel powder of the same volume ratio, its maximum heat load is only 210 W (the corresponding heat flux is 17.6 W/cm2). The multi-composite porous wick shows better evaporation heat transfer performance. After the evaporation heat transfer experiment, the oxidized multi-composite porous wick still shows good wettability. It is inferred that a mixed wetting structure is formed inside it due to the composite structure sintered by three kinds of metal powders, which may be beneficial for improving the evaporation heat transfer performance of the multi-composite porous wick. In addition, the conditions such as lower effective thermal conductivity, more uneven local thermal conductivity, and large pores provided by the unchanged spherical nickel powder may also play a role.

Numerical simulation of heat transfer characteristics of falling-film evaporation of R32/R134a non-azeotropic refrigerant outside a horizontal tube

In response to the restrictions of the international community on the utilization of refrigerants with high GWP, the replacement of high-GWP refrigerants with non-azeotropic refrigerants and reduction in refrigerant charge have gradually become important research topics in academia. In this study, based on the VOF method, a 2D model of a sheet flow pattern was developed by considering the thermophysical properties and multicomponent phase change of a mixture. The falling-film evaporation of a non-azeotropic refrigerant composed of R32 and R134a outside a horizontal tube was numerically simulated. The effects of the spray height (H), tube diameter (d), inlet temperature (Tinlet), Reynolds number (Re), heat flux (q), and mass fraction of R32 in the liquid-phase mixture (MR32_liquid) on the heat transfer coefficient (HTC) were studied. The results indicate that with the increase of circumferential angle (Φ), the local HTC exhibits a trend of decreasing, increasing, decreasing, and increasing in sequence. The largest contribution to the increase in the average HTC comes from the impact and fully developed regions. An increase in H, Tinlet, and MR32_liquid is beneficial to the expansion of the fully developed region, whereas a decrease in d leads to expansion of the fully developed region. An increase in Re does not affect the extent of the fully developed region. The increase in H, Re, Tinlet, and MR32_liquid increases the average HTC, but the effect of Tinlet is not significant. Under the conditions of d = 24 mm, H = 13 mm, Re = 4000, MR32_liquid = 0.5, and q = 10 kW/m2, as the inlet temperature increases from 243.15 K to 283.15 K, the average HTC increases by 2.5%. An increase in q does not affect the average HTC, whereas an increase in d does not improve the average HTC.

Investigation of Heat Transfer and Pressure Drop for R744 in a Horizontal Smooth Tube of R744/R404A Hybrid Cascade Refrigeration System—Part 2: Low-Temperature Region

This paper studies the evaporative heat transfer characteristics of R744 at low temperatures in a horizontal smooth tube as a cascade refrigeration system (CRS) among hybrid cascade refrigeration systems (HCRSs). There is a lack of research on the low-temperature evaporative heat transfer characteristics of R744 under the operating conditions of evaporators used in actual CRSs used in supermarkets. Therefore, this study aims to provide basic data on the evaporative heat transfer characteristics of R744 in the evaporators of refrigerators used in supermarkets. The tube used in the evaporation experiment conducted herein was a smooth horizontal copper tube with an inner diameter and length of 11.46 mm and 8000 mm, respectively. The experimental parameters were as follows: heat fluxes of 11.8–21.3 kW/m2, mass fluxes of 76.3–175.1 kg/(m2·s), and saturation temperatures of −49.8–−30.2 °C. The main results are summarized as follows: the evaporative heat transfer coefficient of R744 can be predicted well by using the correlation formula of Chen at the evaporation temperature of −40 °C in the CRS.

Evaporation Flow Heat Transfer Characteristics of Stainless Steel and Copper Enhanced Tubes

An experimental study was undertaken to study the tube-side evaporation heat transfer characteristics of enhanced tubes and compare their performance with that of smooth tubes. These experiments were conducted in order to determine how R410a evaporates inside smooth and enhanced tubes; for a saturation temperature of 279.15 K; with mass flux values that ranged from 50 to 250 kg/(m2·s); for an inlet quality of 0.2 and outlet quality of 0.8. Enhanced tubes evaluated include herringbone (HB) and helix (HX) designs with microgrooves, composite herringbone dimple (HB/D), composite herringbone hydrophobic (HB/HY), and composite EHT (multiple enhancement character) tubes. Experimental results show that the evaporation heat-transfer coefficient in the Cu-EHTb tube was the highest; its performance was closely related to the increased number of nucleation points that are found inside the tube; however, the performance of the SS-EHT-HB/D was not significantly higher than that of a smooth tube. The best overall capacity for evaporative heat transfer is shown in the SS-EHT-HB/HY and SS-EHT-HX tubes; the SS-EHT-HB/D, Cu-EHTa, and Cu-EHTb tubes had the worst overall capacity among all the tested tubes. Additionally, it was determined that previously reported smooth tube models to determine the evaporation heat transfer coefficient can accurately predict the heat transfer inside a smooth tube. However, when trying to utilize smooth tube models for enhanced tubes, the deviation between experimentally determined heat transfer coefficient (HTC) values and those predicted when using smooth tube models to predict enhanced tube results is ±30%; therefore, smooth tube models are not applicable for use with enhanced tubes. Smooth tube models were modified, and after correction, the deviation between experimentally determined heat transfer coefficient (HTC) values and those predicted when using the modified model for use with enhanced tubes is ±10%. Finally, the effect of the thermal resistance of the tube wall on the overall heat transfer coefficient of a stainless steel-enhanced tube is significant and cannot be overlooked.

Flow evaporation heat transfer of liquefied natural gas in a spiral pipe based on fluid coupling

Abstract In the extraction and usage of liquefied natural gas (LNG) cold energy, the heat transfer research of cryogenic fluid in spiral coil is mostly under the simplified condition of constant boundary temperature or constant heat flux density. This paper focuses on the actual situation of fluid–fluid coupling heat transfer in spiral coil. Under the condition of variable physical parameters, computational fluid dynamics numerical analysis is carried out on the non-customized numerical model through the Realizable k-ε turbulence model, and the influence of LNG flow and evaporation heat transfer characteristics in spiral coil is studied by changing the heat transfer structure. The results show that the heat transfer under non-constant boundary conditions is more in line with the actual heat transfer, and the spiral radius has the greatest influence on the heat transfer coefficient. When the spiral radius increases from 40 to 100 mm, the heat transfer coefficient decreases by 60.14%.

Experimental Study on Effects of Different Water Flow Rates on Heat Transfer Characteristics of Evaporator

Since China set forth the goal of “dual carbon”, studies on heat transfer characteristics of evaporators have been one of the hot spots in the field of refrigeration and heating. This paper conducts an experimental study on the heat transfer performances of evaporators under different flow rates by changing the water flow of the evaporator and condenser. The results shows that on the premise of unchanged water flow of condenser, cooling coefficient of performance (COP) and heat transfer coefficient (HTC) grow with the increases in flow rates of evaporator and temperatures of inlet and outlet water. However, the changes in HTC are relatively smooth; the compressor’s inhalation and expulsion temperatures grow with the increases in water flow rates of the evaporator, and the changes are quite evident. The paper also discusses and analyzes deeply those manifestations, which provides experimental evidence for the experiments and improvement of the evaporator, optimization of evaporator design, and the enhancement of the economic performance of the system.

Evaporation Heat Transfer Characteristics from a Sintered Powder Wick Structure Sandwiched between Two Solid Walls

Evaporation Heat Transfer Characteristics from a Sintered Powder Wick Structure Sandwiched between Two Solid Walls

Evaporative heat transfer of R410A flow in an annular duct under oscillatory wall heat flux

In this investigation, the evaporative heat transfer characteristics of R410A flow in a slender annular duct under an oscillatory heat flux are studied experimentally with emphasis on the influences of mean mass flux, heat flux, and oscillating amplitude and period of heat flux. The applied heat flux is varied in ranges of q = 5,000 to 15,000 W m−2 and the mean mass fluxes of 300 and 400 kg.(m−2s−1) are considered, respectively. The results reveal that increasing either oscillating amplitude or period of heat flux enhances the oscillation of surface temperature and evaporation heat transfer coefficient (HTC). The prevailing influence of heat flux oscillations on the evaporation heat transfer causes notable variations in the vapour quality and liquid film thickness. At a high level of vapour quality, the thickness of refrigerant liquid film on the hot surface is decreased, leading to the increase of the evaporation HTC. Furthermore, the oscillation in the wall temperature is slightly stronger as the period of heat flux oscillation boosts in the range of 120 s to 600 s. Based on the present measured data, some correlations are provided for the evaporation HTC of R410A as the function of the heat flux and vapour quality.

Investigation of Heat Transfer and Pressure Drop for R744 in a Horizontal Smooth Tube of R744/R404A Hybrid Cascade Refrigeration System—Part 1: Intermediate Temperature Region

In this study, the evaporation heat transfer characteristics of an intermediate temperature of R744 in a smooth horizontal tube, when operating as an indirect refrigeration system (IRS) among hybrid cascade refrigeration systems (HCRSs), are evaluated. Studies on the characteristics of intermediate-temperature evaporation heat transfer under the operating conditions of evaporators used in actual refrigeration systems, such as IRS, cascade refrigeration systems, and HCRS, used in supermarkets are lacking. Thus, this study provides basic data on the characteristics of evaporation heat transfer of R744 in the evaporators of refrigerators used in supermarkets. The tube employed to the evaporation experiment in this study was a horizontal smooth copper tube with a length of 8000 mm and an inner diameter of 11.46 mm. The experimental variables were measured over a wide range of mass flux of 200–500 kg/(m2·s), heat flux of 10–40 kW/m2, and saturation temperature of −40–0 °C. The main results are summarized as follows: (1) The application of Kandlikar’s correlation formula at an evaporation temperature of −20 °C in an IRS helps in a good prediction of the R744 evaporation heat transfer coefficient. (2) The pressure drop according to the heat and mass flux showed the same heat transfer coefficient trend, but the pressure drop at saturation temperature was different from the trend of heat transfer coefficient.

Evaporation heat transfer coefficient and frictional pressure drop of R600a in a micro-fin tube at low mass fluxes and temperatures

In this study, the evaporation heat transfer characteristics of R600a in micro-fin and smooth tubes are investigated under the operating conditions of domestic refrigerators. The evaporation heat transfer coefficient and frictional pressure drop of R600a were measured and analyzed at various vapor qualities ranging from 0.2 to 0.9, mass fluxes from 20 to 40 kg m−2 s−1, saturation temperatures from −25 to −10 °C, and heat fluxes from 9 to 15 kW m−2. The evaporation heat transfer characteristics of R600a in the micro-fin and smooth tubes were compared in terms of the enhancement factor, penalty factor, and enhancement parameter. The micro-fin tube was preferable under low mass flux conditions, resulting in a higher enhancement parameter. Additionally, because of the poor prediction of the existing correlations, new empirical correlations for the evaporation heat transfer coefficient and frictional pressure drop of R600a in the micro-fin tube were proposed with high accuracy. These results can be used to design evaporators for domestic refrigerators under low saturation temperature and mass flux conditions.

Evaporation heat transfer and flow characteristics of vertical upward flow in a plate-fin heat exchanger

This study experimentally investigated the evaporation heat transfer and two-phase flow characteristics of a vertical upward flow using R1234ze(E) and R32 as test refrigerants in a plate-fin heat exchanger with offset fins. The adiabatic and evaporation flows of R1234ze(E) were visualized using a high-speed camera. Owing to the asymmetry of the inlet and outlet of the channel, a non-uniform velocity profile in the channel-width direction was observed under both adiabatic and evaporation flows. Evaporation flow accompanied by nucleate bubble generation at the lower area of the channel and the occurrence of a dry area at the upper side of the channel were observed. The heat transfer coefficients of R1234ze(E) and R32 were measured, and the effects of mass flux, quality, and heat flux on heat transfer were clarified. The heat transfer was strongly affected by forced convection at higher mass fluxes and quality whereas nucleate boiling dominated at higher heat fluxes and lower quality. The difference in the heat transfer characteristics at local positions in the channel was also investigated by measuring the local wall temperatures. Comparing refrigerants, the heat transfer coefficient of R32 was higher than that of R1234ze(E), regardless of the mass flux and quality, especially at lower mass flux conditions.