Evaporation Heat Transfer Performance Research Articles

Analysis of thermal performance characteristics of rotating packed beds using response surface methodology

Cooling towers play a crucial role in various industries to dissipate the heat of circulating water. This process involves the direct contact of atmospheric air and hot water from the condenser within a porous packed bed. The cooling towers suffers from their large size which results in escalating construction and maintenance expenses. In the present work, a rotating packed bed has been investigated experimentally for its evaporative heat transfer performance and is proposed as an alternative to cooling towers. In the chemical and processing sectors, rotating packed beds are being explored as a potential alternative to conventional distillation towers due to their process intensification capabilities. The principal reason behind this adaptation is the enhanced mass transfer rates achievable by rotating packed beds in a compact size. The aim of this study is to lay the foundation for rotating packed beds to achieve similar process intensification in various heat transfer applications. For this purpose, the parameters that describe the performance of a cooling tower such as air pressure drop, cooling range, heat rejection rate from water, cooling effectiveness, and Merkel number were selected to assess the performance of the rotating packed bed. The effect of various factors such as mass flow rate of air, mass flow rate of water, water inlet temperature, and rotational speed on the performance parameters was observed. Response surface methodology was used for the design of experiments, development of the regression equations, and optimization of the performance parameters of rotating packed bed. The cooling effectiveness value of up to 0.59 and the Merkel number of up to 1.88 were achieved in the rotating packed bed.

Numerical study on heat transfer and evaporation vaporization performance of solar assisted heat pump regenerative evaporator based on dual-phase change coupled heat transfer

Using phase-change slurry (PCS) as the working medium for solar energy systems, for example, in solar-assisted heat pump (SAHP), offers numerous advantages such as photovoltaic (PV) and photothermal (PT) energy generation and heat supply. The PCS experiences a dual-phase change coupled heat transfer during the heat release and refrigerant vaporization, as investigated through simulation. This study confirmed a coupled heat transfer with dual-phase change. It was found that there is more vapor at the upper coupling interface than at the lower coupling interface, and the difference is obviously enhanced when the flow rate decreases from 0.4 m s−1 to 0.1 m s−1 or the temperature increases from 12 °C to 16 °C. The increased rate of flow or decrease in refrigerant temperature is associated with a reduced equilibrium concentration at the lower coupling interface. Nevertheless, an increase in the flow rate at the lower coupling interface causes a decrease in the equilibrium concentration, while raising the temperature increases the concentration. When the flow rate increases from 0.1 m s−1 to 0.4 m s−1, the heat transfer coefficient at the upper/lower coupling interface increases from 159.77 W m−2 K−1/292.58 W m−2 K−1 to 593.54 W m−2 K−1/654.36 W m−2 K−1, and the heat transfer enhancement ratio reaches 45.39% and 9.29%, respectively. When the refrigerant temperature increases from 12 °C to 16 °C, the heat transfer coefficient at the upper/lower coupling interface decreases from 579.16 W m−2 K−1/572.91 W m−2 K−1 to 329.44 W m−2 K−1/365.67 W m−2 K−1, respectively. Increasing tilt angle of the pipe within a moderate range is a potential solution to enhance heat transfer and improve heat transfer uniformity.

Experimental and numerical investigation on heat transfer performance of a single-tube six-fin tube evaporator

Tube evaporators are widely used in cold storage, the single-tube two-fin tube evaporator (2-FTE) has the problems of poor heat transfer performance and low tube utilization. In this paper, a numerical simulation method is used to optimize the structure of 2-FTE, and a single-tube six-fin tube evaporator (6-FTE) structure is designed to achieve an increase in the cooling capacity per unit mass of the tube and heat transfer efficiency. Based on this, the heat transfer performance test bench of the tube evaporator is constructed, and the heat transfer performance of 2-FTE and 6-FTE is compared and tested according to the evaluation indices of cooling capacity, fin efficiency and cooling rate in the test environment from −32 °C to −6 °C. The results show that 6-FTE saves between 10.6 and 17.1% of tube consumption compared to 2-FTE for the same cooling capacity. The cooling capacity per unit tube length of 6-FTE increased by 43.9–64.7%, the cooling rate increased by 28–41%, the overall fin efficiency increased by 3.5%, and the heat transfer performance was better than that of 2-FTE, which was consistent with the simulation conclusions and in line with optimization goals. Therefore, the 6 FTE structure proposed in this paper can be an effective solution to save the manufacturing costs of refrigeration equipment for cold storage.

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.

Research Progress on Direct Contact Evaporation Heat Transfer between Two Immiscible Working Fluids

AbstractThe direct contact evaporation heat transfer of two immiscible fluids can realize low‐temperature differential heat transfer, which is an important method to improve the heat transfer coefficient. This article focuses on the dispersed phase of organic working fluids and the continuous phase of immiscible inorganic fluids and summarizes the research on direct contact evaporation heat transfer in the past 15 years. The main research methods include theoretical analysis, numerical models, and experimental research. The evaporation process of a single droplet mainly involves bubble droplet growth, droplet configuration and discriminant, droplet shape change, rising speed and path, etc. In practice, dispersed phases mostly exist in populations, and the research mainly focuses on collisions, coalescence, and rupture between droplets and bubbles, as well as on the number and size distribution. The volumetric heat transfer coefficient is used to reflect the heat transfer capacity of the heat exchanger. The factors affecting evaporation heat transfer performance are complex, and increasing the uniform mixing of bubbles is an important method to improve the heat transfer coefficient. In the future, direct contact evaporation heat transfer is expected to be promoted to multiple fields.

Experimental study on low pressure heat transfer properties of water evaporator with screen separation element

Water evaporator is often used in spacecraft thermal management as heat sink. In a water evaporator, heat is transferred from the microchannels of cooled glycol liquid to the external water, which causes the phase transition of water near external fins. Due to the severe weight requirements of hypersonic vehicles, it is very important to study the heat transfer performance and water utilization of water evaporator. The present study experimentally investigated the boiling heat transfer of water evaporator on external rectangular fins (RF) at low pressure, and the heat transfer of cooled glycol liquid in microchannels with offset strip fins (OSF). At the same time, the experiment explored the effects of three types of screen devices on the heat transfer performance and water utilization efficiency of water evaporators. The heat transfer experiments of water evaporator were conducted at the pressure of 11 kPa. The inlet temperature of hot glycol liquid was controlled 85 °C, and the mass flow rate were at 1500 kg/h and 2500 kg/h, respectively. The experimental results show that when the mass flow rate of ethylene glycol is the same, the water consumption of the water evaporator significantly decreases and the water utilization efficiency increase with the increase of screen density, but it can weaken the boiling heat transfer performance. In addition, the empirical correlation of boiling in low-pressure is modified, and the maximum deviation is within ±10%. The screen separator element and the new modified correlation provide a reference for the design of water evaporator.

Multi-objective optimization design of a plate–fin water evaporator in a low-pressure environment based on the MO-SHERPA algorithm

The aim of this study was to investigate the optimal heat transfer performance and structure of airborne water evaporators in low-pressure environments. To achieve this, a multi-objective optimization design was performed on a plate-fin water evaporator with offset strip fins, operating in a low-pressure environment at an absolute pressure of 7 kPa. The multi-objective Simultaneous Hybrid Exploration that is Robust, Progressive and Adaptive (SHERPA) (MO-SHERPA) algorithm was used to optimize the structure of the plate–fin water evaporator under a low pressure of 7 kPa. There are three conflicting optimization objectives, including comprehensive performance factor (JF), entropy generation (Sgen), and weight. The optimal Pareto frontier design points were compared and analyzed based on the three evaluation indicators. The optimization results showed that compared with the initial design point, the JF increased by 32.14% and the weight decreased by 48%, which indicated that the heat exchange performance has been significantly improved, and the weight of the heat exchanger was also reduced. The internal and external flow fields of the water evaporator, including temperature, pressure, and volume fraction of vapor, were qualitatively compared to prove the optimization effect. In addition, the influence of structural parameters on the heat transfer performance of the water evaporator were analyzed. The method proposed in this study can effectively optimize the fin structural design parameters, providing valuable guidance for the structural design of water evaporators operating in low-pressure environments.

Research and Improvement of Heat Transfer Performance of Plate and Shell Evaporator

Evaporator plays a key role in indirect refrigeration system, so improving the efficiency of evaporator is of great significance to improve the overall performance of refrigeration system. For the plate and shell evaporator, the reliability of the model is proved by numerical simulation of the existing evaporator and comparison with the experiment. An improved scheme is proposed for the existing evaporator, and the improved model is simulated and analyzed. The result shows that compared with the previous model, the improved model improves the heat exchange efficiency, reduces the pressure drop, and improves the overall efficiency of the evaporator.

High energy-density and power-density cold storage enabled by sorption thermal battery based on liquid-gas phase change process

Cold storage is essential for the preservation of food/medical goods, energy-saving of air conditioning, and emergency cooling. However, conventional cold storage in the form of sensible heat or solid-liquid latent heat suffers from the low energy density and large cold loss during long-term storage. To address the problem, a scalable sorption thermal battery (STB) with storage capacity of 30 kWh is designed and fabricated for realizing high energy-density and power-density cold storage by proposing sorption-induced liquid-gas phase change strategy. The STB utilizes the bonding energy of sorption working pair of zeolite 13X-water to achieve cold storage during charging phase, and utilizes the liquid-gas evaporation heat of water to realize cold production during discharging phase. The cold-storage performance of the STB prototype during charging/discharging processes is experimentally investigated at different operating conditions. By optimizing the assembled structure of solid-gas reactor and enhancing the heat transfer performance of liquid-gas evaporator, the STB exhibits high cold energy density up to 114.92 Wh/kg and 26.76 kWh/m3, and high power density of 455.62 W/kg and 106.10 kW/m3, over the conventional cold storage technologies. Correspondingly, the STB demonstrates a good performance in thermal management of data centers with cooling power as high as 150 kW. Moreover, the STB has the distinct advantage of near-zero cold energy loss in long-term cold storage. The proposed scalable sorption thermal battery based on sorption-induced liquid-gas evaporation heat of liquid offers a promising route to realize high energy density cold storage.

Effect of cooling water flow on heat transfer performance of horizontal tube spray falling film evaporator in ORC system

Organic Rankine cycle (ORC) uses low-temperature heat source to realize thermoelectric conversion, which is an important way to realize the effective recovery and utilization of waste heat. Based on the horizontal tube spray falling film evaporator, an orc power generation system with R245fa as the circulating working medium is built to study the influence of the change of cooling water flow on the heat transfer performance of the evaporator. The results show that under the condition of constant heat source temperature, the evaporation pressure and condensation pressure first decrease and then increase with the increase of cooling water flow, while the total heat transfer coefficient outside the evaporator tube first increases and then decreases. When the cooling water flow is greater than 3100 L/h, it has little effect on the heat exchange system of the evaporator, and it is more appropriate to keep the cooling water flow at about 3100 L/h. By studying the influence of cooling water flow on the heat transfer performance of evaporator, an important experimental basis is provided for the design and optimization of ORC cooling system.

Evaporation behaviors of R32/R1234ze(E) mixtures on a copper surface using molecular dynamics simulations

As an essential heat transfer mode, evaporation has received much research attention. The evaporation characteristics of a mixed refrigerant are greatly affected by the fraction of each component. Hence, it is essential to analyze the effect of the composition of the mixed refrigerants with different mass fractions on evaporation. Here, the evaporation behaviors of R32/R1234ze(E) mixtures with various mass fractions on a copper surface were studied by molecular dynamics (MD) simulations. Prior to conducting the evaporation simulation, several thermophysical properties were calculated to verify the reliability of the potential function used in MD simulations. Then, the evaporation behaviors of five different R32/R1234ze(E) mixture systems (mass fraction xR32 = 1, 0.67, 0.50, 0.33, 0) on the copper wall were simulated to determine the effect of the composition ratios on the evaporation. The results show that the R32 molecules are easier to evaporate than R1234ze(E), and an increased fraction of R32 increases the evaporation rate of the mixed refrigerant systems. Furthermore, the intermolecular interaction energies were calculated to explain the simulation results. The present study is also helpful for understanding the previous experimental finding that a high mass fraction of R32 can enhance the evaporation heat transfer performance of mixed refrigerants.

Study on the Coupling Effect of Heat Transfer and Refrigerant Distribution in the Flat Tube of a Microchannel Evaporator

Refrigerant maldistribution severely deteriorates the heat transfer performance of a microchannel evaporator. Compared with the refrigerant distribution among flat tubes along the header, refrigerant distribution among microchannels in the flat tube in the airflow direction has barely been paid attention. In this paper, a heat transfer mathematical model of a microchannel evaporator’s flat tube composed of vertically placed parallel microchannels in the airflow direction was developed. The Refrigerant distribution among the microchannels was evaluated and its influence on heat transfer between air and refrigerant was analyzed. The results showed that the refrigerant distribution and heat transfer performance between air and refrigerant were interrelated and interacted with each other. The temperature of the air leaving the microchannel evaporator changed along the microchannel because of uneven refrigerant distribution among the microchannels, and the air temperature difference between air leaving out of the bottom and the top of the evaporator was approximately 2.13 °C. Ignoring the heat transfer from adjacent microchannels will lead to a small heat transfer deviation for the flat tube; thus, heat transfer among microchannels can be neglected.

Evaporation of ultra-thin water film on hot spot with nanopillar array

For the ultra-thin film evaporation, a hot spot was formed by a local dry out, resulting in the heat transfer deterioration. In this work, the stable evaporation of ultra-thin water film on hot spot with designed nanopillar structure was studied based on the non-equilibrium molecular dynamics (MD) simulation. The variation of molecular motion trajectories, evaporation rate and number of migrating water molecules in different regions were recorded over time in the simulation. The results show that the nanopillar array on the hot spot can attract the surrounding liquid to wet the overheated region due to the strong solid-liquid interaction, both the evaporation stability and heat transfer performance can be improved. By comparing three surfaces with different nanopillar arrays on hot spot, it shows that the evaporation rate is one important factor affecting the amount of liquid replenishment: with the increasing number of nanopillars, both the evaporation performance and liquid replenishment capacity are improved. Moreover, the effect of two designed nanostructure configurations on the evaporation performance was investigated. Compared to the configuration of only nanopillar array on the hot spot, the configuration of nanopillar array on the whole surface has a lower temperature for the nanopillars and delays the occurrence of dry out, while the averaged evaporation rate may be reduced as well. Therefore, there is a trade-off between the surface temperature reduction and evaporation rate enhancement. The findings in this study can provide a guidance for the thermal design for the hot spot in the chip cooling technique under the high heat flux application.

Three-dimensional lattice Boltzmann investigation on pore scale liquid–vapor distribution patterns and heat transfer performance of a loop heat pipe heterogeneous porous wick evaporator

The pore scale numerical simulations on evaporation of a three-dimensional heterogeneous porous wick capillary evaporator used in a loop heat pipe (LHP) are realized through using a lattice Boltzmann method with liquid–vapor phase change. The dynamic liquid–vapor interface characteristics are obtained. The steady-state wick state and heat transfer performance as well as the influences of wick porosity and surface wettability are discussed. It is found that as heat flux increases, the wick state changes from the fully saturated state at low heat fluxes, to the partial dryout state at medium heat fluxes, and finally to the complete dryout state at high heat fluxes. The effective heat transfer coefficient of evaporator firstly grows slowly for the saturated wick, then increases rapidly and reaches a maximum value at partial dryout wick state, but afterwards drops sharply when the wick starts to be complete dryout. With the decreasing of wick porosity and contact angle, the heat flux to achieve the maximum effective heat transfer coefficient is increased and the complete dryout state is postponed. In the present range of parameters, the maximum effective heat transfer coefficient is improved by reducing wick porosity, while it is barely affected by contact angle.

Improved Heat Transfer Performance of Falling Film Evaporator for Desalination Industry by Balanced Liquid Distribution

Horizontal-tube falling film evaporators (HFE) are widely used in the thermal desalination industry due to their increasing advantages. The liquid distribution state of the tube bundle directly affects the heat transfer performance of the evaporator. To improve the uneven liquid distribution among tube rows, a balanced distribution structure (BDS) that can balance the liquid distribution state in the HFE was proposed. A general distributed parameter model was built to describe the steady-state performance of the HFE with and without the BDS, taking into account different liquid distribution of odd and even rows. The model is applied to simulate the multieffect distillation (MED) system of a desalination power plant, and the numerical results show a good arrangement with the operating results. The results indicate that the BDS slightly improves the overall heat transfer performance and the total evaporation rate. This work and the designed BDS certainly could help in the design and optimization of HFE for desalination processes.

Theoretical and experimental study on liquid film thicknesses of unsteady slug flows in a capillary tube

Liquid film thickness is an important parameter for predicting evaporation and condensation heat transfer performance of two-phase slug flow in a capillary tube. Under flow boiling condition, the bubble of two-phase slug flow is elongated and accelerated. In the present study, the effect of acceleration on the liquid film thickness of unsteady two-phase slug flow is theoretically and experimentally investigated. It is first theoretically found that the accelerated slug flow is affected by the dimensionless numbers of capillary number (Ca), Weber number (We), and Bond number (Bo). The relation of the dimensionless liquid film thickness with the capillary number, Weber number, and Bond number for unsteady accelerated slug flows was obtained. The liquid film thicknesses of two-phase slug flows under acceleration condition were directly measured using laser focus displacement metering technique. Circular capillary tube with inner diameter of 1 mm was used for the test tube, and water, ethanol and FC-40 were used as working fluids. The novel prediction correlation of liquid film thickness for accelerated slug flows was proposed and it well predicts all the experimental data for all conditions. As the Bond number increases, the liquid film thicknesses are gradually shifted in the direction of decreasing film thickness due to acceleration effect. At a fixed capillary number, initial liquid film thickness becomes thinner with increasing Bond number.

Study on evaporation heat transfer performance of composite porous wicks with spherical-dendritic powders based on orthogonal experiment

Based on our previous research on the composite porous wick with spherical-dendritic powders, this paper further adopts the orthogonal experiment to study the effect of the combination and the mixing ratio of the spherical and dendritic powders, and the particle size and volume ratio of the pore former. In order to obtain the optimal prepared parameters of the composite porous wick with spherical-dendritic powders, nine porous samples are prepared based on the orthogonal experiment, and their thermal physical properties are measured and analyzed. Five types of operating curves for temperature and evaporating mass are observed in the evaporation heat transfer experiment. The metal powder materials have significant effects on all the considered physical properties in this paper, but the mixing ratio of metal powders only has a significant effect on the critical heat load and the evaporator wall temperature. The particle size and volume ratio of the pore former also have certain influence on these indexes, except that its particle size has no influence on porosity. The highest critical heat load is 530W and occurs in the composite porous wick with the spherical nickel and the dendritic copper powders with the volume ratio of 3:7, and the particle size of 30-45 μm and the volume ratio of 40% for the pore former, which are not the conditions for the lowest effective thermal conductivity. Low evaporator wall temperature and high critical heat load are required for the excellent performance of composite porous wick. High volume ratio of dendritic copper powders is beneficial to increase the quantity of small pores with high evaporating rate, so as to increase the critical heat load and reduce the evaporator wall temperature. This study can guide the parameter design of the composite porous wick with spherical-dendritic powders to improve its performance.

Heat transfer enhancement of a multilayer graphene coating surface

This study examined improvements to evaporation and conduction heat transfer from the use of a multilayer graphene-coated surface. Two surface modification methods, namely the chemical vapor deposition and sol-gel methods, were used to apply multilayer graphene onto copper plates. The results indicated that compared with a plain copper surface, the graphene-coated surface provided superior performance in both evaporation and conduction heat transfer. Compared to the two coated methods, sol-gel method exhibited superior evaporation heat transfer performance because of the surface wetting behavior. However, the conduction heat transfer performance had significantly increased with the chemical vapor deposition method as surface temperature increased to approach saturation temperature.

Evaporation heat transfer characteristic of porous wick in an open capillary evaporator

An open capillary evaporator is proposed to study the evaporation heat transfer performance of porous wick. Both temperature and evaporating mass are measured in this system. Besides, the temperature and relative height of the supplied fluid can be adjusted. A composite porous wick is tested through the constant and variable heat load operation, and it reaches a maximum heat load of 320 W, which corresponds to a heat flux of 25.5 W/cm2. Two types of the evaporator wall temperature curve are observed, and the effect of the initial condition is considered. Intermittent fluid supply appears and causes a temperature overshoot of the evaporator wall. But continuous intermittent fluid supply and small amount of fast-replenishing fluid does not cause the temperature overshoot. The influence of adding water on temperature overshoot is also studied. The boiling and evaporation occur at different heat loads, and the two heat transfer modes coexist at moderate heat load. The transition in heat transfer modes leads to that the evaporation heat transfer coefficient first increases and then decreases with the increase of heat load. This study is helpful to analyze the evaporation heat transfer performance of porous wick and understand the operating characteristics of loop heat pipe.

Molecular dynamics simulations and mathematical optimization method for surface structures regarding evaporation heat transfer enhancement at the nanoscale

Surfaces with nanostructures can significantly enhance the evaporation heat transfer process. Most previous investigations on the designed surface structures using the empirical way to find better geometries. This paper proposes an optimization method for different types of nanostructured surfaces to seek optimal structures mathematically under the given heat transfer area. In this paper, three defined types of nanostructured surfaces are discussed: concave, convex and concave-convex. Firstly, a positive correlation between the evaporation heat transfer performance and the defined sectional area of the liquid phase is obtained by molecular dynamics simulations of the empirically designed nanostructured surfaces. Then, the optimization of the surface geometry converts to a mathematical problem which to solve the maximum sectional area of the liquid phase. This method is used to optimize the geometries of the concave and convex surfaces. Moreover, the molecular dynamics simulation results indicate that the optimal surfaces conduce to better heat transfer performance than the normal concave, convex and concave-convex ones. Finally, to explain the mechanism, the investigations of the atomic potential energy distributions of liquid atoms, the interaction energy between solid and liquid atoms, the solid-liquid interfacial thermal resistance and the morphology of the liquid-vapor interface are done.