Liquid Heat Transfer Research Articles

Atomistic Investigation of Viscoelastic Nanofluids as Heat Transfer Liquids for Immersive-Cooling Applications

Atomistic Investigation of Viscoelastic Nanofluids as Heat Transfer Liquids for Immersive-Cooling Applications

Numerical study on flow pattern transition, liquid film and heat transfer for R1233zd/FC72 mixtures across tube bundles

The non-azeotropic mixtures phase-change flow across tube bundles is widely used in the refrigeration industry. A simulation model of R1233zd/FC72 mixtures is presented to study flow pattern transition and heat transfer across tube bundles. Volume of Fluid (VOF) model is used to determine the flow pattern. An experiment is conducted to validate the simulation model with average deviation of 8.97%. The results showed that the flow pattern transition appears for various conditions. In addition, the liquid film thickness and heat transfer are analyzed, focusing particularly on the impact of various mixture components. With increasing vapor quality, the flow pattern transfers from sheet flow to semi-annular flow and then to droplet flow. Sheet to semi-annular flow transition occurs for a vapor quality of 0.4–0.6, and semi-annular flow transfers to droplet flow for a vapor quality of 0.7–0.8. For sheet flow, the local heat transfer coefficient on the tube surface is the highest at 90° (circumferential angle) from the summit point of the cross-sectional tube circle due to the thinnest liquid film. The liquid film turns thinner with increasing mass fraction of R1233zd and vapor quality. Increasing the mass fraction of R1233zd and vapor quality promotes heat transfer. However, the flow pattern transfers from sheet flow to the other ones, the heat transfer coefficient starts to decrease. The heat transfer coefficient is the highest for a vapor quality of around 0.4–0.5 and a mass fraction of R1233zd of around 0.7–0.9.

Air to liquid heat transfer coefficient experimental comparison between silicon carbide and glass shell and tube heat exchangers in a pilot plant scale

ABSTRACT Instead of the expected 3.8–5.4% increase in the heat transfer coefficient due to the better thermal conductivity of silicon carbide tubes compared to glass tubes, the observed increase was 18–22% for 150–275 kg·h−1 airflow and 6 kg·s−1 propane-1,2-diol coolant in tubes. This additional 15–17% increase is probably due to local flow turbulisation due to the roughness of the sintered carbide of 4–10 µm, which unfortunately also causes a 17–24% higher air pressure drop. The hand calculation model used underestimates the heat transfer coefficient by −2% to 10%, which is better than CHEMCAD 8 modeling results.

Coupled CFD modeling and thermal analysis of multi-layered insulation structures in liquid hydrogen storage tanks for various vapor-cooled shields

The present study aims to propose a coupled numerical model for analyzing the influence of the configuration of the vapor-cooled shield tubes on insulation performance. The numerical model is developed to calculate the effective thermal conductivity of multi-layer insulation (MLI) and rigorously evaluated. Specifically, the tank evaporation simulation software BoilFAST is manually coupled with the CFD commercial software of ANSYS FLUENT (2020 R2) to accurately determine the mass flow rate and temperature of boil-off gas in a tank, which is then used as inputs for the CFD simulations. This coupling methodology can be used for reliable calculations of boil-off losses for liquid hydrogen and heat transfer inside the MLI structures. Results demonstrate that the estimated effective thermal conductivity agrees with the earlier published data and that increasing the diameter and number of tubes enhances heat exchange between the VCS tubes and MLI structures. Also, the heat flux into the tank is relatively reduced by 32.04 % by increasing the diameter of the tube and 49.14 % by adding the number of tubes. Moreover, the maximum temperature difference of the VCS is estimated to be less than 1 K, indicating nearly uniform distributions of the VCS.

Research on mercury re-release model in wet flue gas desulfurization (WFGD) in coal-fired power plants

Mercury (Hg), a toxic heavy metal, has triggered regulations for emission control from coal-fired power plants. It has been found that mercury re-release occurs in WFGD slurry, which is affected by WFGD process parameters. The coal co-combustion with bromide technology proves to be an efficient approach for enhancing mercury removal ratio but the accumulation of introduced bromine in WFGD slurry would impact the re-emission behavior of mercury significantly. Model study is efficient method to reveal the physicochemical processes in WFGD systems. However, previous model studies were carried out under Hg2+ contained simulated slurry systems without consideration of Hg2+ transformation process from gas to liquid phase, and co-combustion with bromide technology was not considered. Here, a model which encompasses droplets motion sub-model, gas–liquid heat transfer sub-model, gas–liquid mass transfer sub-model and the mercury re-release kinetic sub-model is developed and validated against experimental data. The effects of bromine concentration, slurry temperature, pH value and S(IV) concentration were investigated. It was found that introduced bromine has significant inhibitory effects on the re-release of mercury. When the slurry temperature is higher, the absorption rate of Hg2+ is lower and more Hg0 is released. Moreover, the increase in the slurry pH value and S(IV) concentration has certain inhibitory effects on the re-release of Hg0 in WFGD. Above all, the developed model has high prediction accuracy for the absorption of Hg2+ and the re-release of Hg0 in WFGD which is expected to be beneficial for guiding the operation of WFGD systems.

HFE-7100 spray cooling under microgravity: Liquid film dynamics and heat transfer

Spray cooling under microgravity is not only motivated by space applications but also by the essential pursuit of the exploration spirit facing the unknown. This study was the first series of scientific experiments conducted at the China New Microgravity Experiment Facility with Electromagnetic Launch. In this study, the liquid film dynamics and heat transfer characteristics of HFE-7100 spray, including liquid film morphology, wetted area, equivalent diameter, and velocity under different inlet pressures and heat fluxes, were quantitatively captured on smooth and rough silicon surfaces under microgravity. Several classic scenarios of liquid films, including spreading, bifurcating, bridging, fusiform, tadpole-shaped, and isolated liquid film, were recognized for the first time. The morphologies of the liquid films and the heat transfer performance under smooth and rough surfaces showed insignificant differences under normal and microgravity conditions. In addition, the dimensionless groups (Weber number and Reynolds number) were updated corresponding to approximately 92.4 % of the total We samples and approximately 96.4 % of the total Re samples ranging from 0.01 to 1 and 10 to 300, respectively, regardless of the different coolants (HFE-7100 and HFE-7000), pressures, nozzle-to-surface heights, heat fluxes, surface conditions, orientations, and gravities (1 g and µg).

Redesign of Heat Transfer Pipes in Radiators, Air-Conditioners and Refrigerators for Higher Efficiency

Heat transfer is a surface phenomenon. Instead of straight hollow rectangular cross section tubes with heat transfer liquid passing through it, a triangular cross section tubes having the tip of the triangle facing front to receive the flow of air is the proposed new geometrical design. With minimum material maximum heat transfer as a whole is the aim of the new Radiator design.

Study on heat and mass transfer behavior of coupled electrochemical reaction in porous structure of proton exchange membrane fuel cell

Proton exchange membrane fuel cell (PEMFC) is widely used in transportation, aviation and energy storage due to its unique advantages. Improving the performance of fuel cells depends largely on effective hydrothermal management. In this work, a multi-physical field coupling model based on the lattice Boltzmann method (LBM) is established to analyze the oxygen transport, liquid water transport, heat transfer and electrochemical reaction, mainly considering the difference between the overpotential of the cell and the surface wettability of the structure. The results show that the overpotential is closely related to the reaction process. The rate of water production is significantly accelerated by increasing the overpotential, and the breakage and merging of water clusters are observed. The increase of overpotential will lead to more energy loss in the form of heat, thus increasing the heat production. Enhancing the hydrophobicity of the structure has a positive effect on the performance of the fuel cell, and the possibility of local hot spots in the low-hydrophobic structure is greater. It is feasible to improve water management and fuel cell performance through wettability design.

Characteristics of heat and mass transfer in moist air-ionic liquid desiccant bubbly systems: Parametric assessment of regeneration performance

Ionic liquids (ILs) have recently become an increasingly popular working medium for liquid-desiccant deep dehumidification as applied to low-humidity industries and, in particular, the use of IL desiccants in bubble columns regeneration has been proven to be an effective way to promote their deep dehumidification potential. The present work considers a moist air-IL desiccant bubbly deep dehumidification/regeneration system, focusing specifically on the heat and mass transfer characteristics of regeneration in bubble column. An experimental apparatus is constructed and used to explore the effects of key operating parameters (i.e., liquid height H, superficial velocity vs, solution temperature Ts) on regeneration performance. Furthermore, a semi-analytical model of bubbly regeneration heat and mass transfer is developed and used to perform parametric assessments. The results indicate that elevating the liquid height, as well as higher superficial velocities and solution temperatures all contribute to improved regeneration rates, each with distinct influencing mechanisms. A higher superficial velocity enhances the volumetric transfer coefficient through a combined effect (i.e., gas–liquid specific interfacial area, heat and mass transfer coefficients). The effect of the solution temperature on the volumetric transfer coefficient is mainly reflected in the heat and mass transfer coefficient as determined by the transfer potential difference, while the hydrodynamics play a relatively minor role in the system of interest. Bubbly regeneration differs from deep dehumidification in terms of the volumetric transfer and heat/mass transfer coefficients. Notably, the Lewis number of bubbly regeneration is generally between 0.6–1. In practical applications, it is necessary to consider the air-side pressure drop, droplet entrainment, and low-grade thermal energy utilization. Under recommended conditions (e.g., H = 0.3 m, vs = 3.0 cm/s, Ts = 60 °C), a regeneration rate of 49 g/h and moisture effectiveness of 79 % can be achieved. This study provides guidance for the design of moist air-IL desiccant bubbly deep dehumidification/regeneration systems.

Numerical investigation of heat and mass transfer characteristics of steam condensation containing non-condensable gas in vertical corrugated tube

A numerical simulation of steam condensation containing non-condensable gas is conducted in this paper to analyze the heat and mass transfer characteristics in vertical corrugated tube. The species transport model, Euler liquid film model and diffusion-balance model with Dehbi factor to correct the diffusion effect are applied to simulate the mass and energy transfer processes. The effects of different corrugated tube structural parameters and inlet steam parameters on steam condensation rate, liquid film distribution and heat transfer are simulated. The results show that vortex and accumulated non-condensable gas in the corrugated region significantly influence condensation heat transfer, which are closely related to corrugation height (H=0–5 mm) and corrugation length (L=13, 16, 19, 22, and 25 mm) of the corrugations, as well as inlet velocity (Vin = 5, 7, 9, 11, 13, and 15 m·s−1) and inlet steam content (Ws = 0.6, 0.7, 0.8, 0.9, 0.99, and 1). The total heat transfer coefficient varies linearly with inlet velocity when inlet velocity increase from 5 m·s−1 to 15 m·s−1, which increase 89.6–108.1 %; total heat transfer coefficient increases almost exponentially when inlet steam content increase from 0.6 to 1, which increase 389–460 %; and the best corrugated tubes are those with corrugation height of 0.075 times diameter of tube and corrugation length of 0.325 times diameter of tube.

Improvement in heat transfer and flow pattern of sprayed falling on horizontal tubes with rib structure for a sewage source heat pump

The falling film flow and heat transfer characteristics are critical to improve the performance of spray heat exchangers in a sewage source heat pump (SSHP), which is influenced by the operating and structural parameters. Therefore, considering the unique flow patterns of wastewater falling film caused by the oily content, and the influence of heat exchange tubes with surface structures on flow and heat transfer, two types of heat exchanger tubes with axial and circumferential ribs were designed and the falling flow over tubes were investigated through CFD method using VOF model. The falling film flow pattern, liquid film coverage and heat transfer coefficient (HTC) outside the ribbed heat exchange tubes were investigated, and the influence of rib structures and Re were analyzed. It was found that oily wastewater liquid film spreaded differently depending on the rib structures, and circumferential ribs of case 2 improved the flow pattern by increasing film coverage, which obviously affected the tube HTC. At higher Re, the ribbed tubes displayed higher potential in better heat transfer performance. Therein, the circumferential ribbed tubes showed highest average HTC, with up to 31.9 % higher than smooth tubes, which can be further enhanced by increasing Re. This study provides a foundation for enhancing the heat transfer performance of spray heat exchangers in sewage source heat pumps through the design and modification of tube surface structures.

Energetic and economic analysis of a novel concentrating solar air heater using linear Fresnel collector for industrial process heat

Industrial processes share a relevant portion of global energy consumption. A large variety of high energy-demanding industrial processes need hot air in the medium temperature range, provided by natural gas combustion or by electricity. Hot air is used as a medium in a large variety of processes such as drying, curing, and thermal treatments of several products and materials. Heat can be provided by solar thermal technologies aimed at the sustainable industry. Non-concentrating flat plate type solar collectors can directly heat air up to 100 °C while higher temperatures can be achieved using linear concentrating technology. In this study an innovative system using linear Fresnel collectors directly provides hot air for the industry up to 350 °C, avoiding the need for liquid heat transfer fluids. Accordingly, the installation is simplified, and lower installation and maintenance requirements are expected compared to other solar technologies. The present studies provide an energetic and economic analysis of the concentrating solar air heater, considering a medium-scale benchmark as a reference case in representative European locations. The results indicate that the solar technology here presented can be economically competitive with conventional natural gas heating, having a huge potential for fossil fuel source replacement in hot air based industrial processes.

A parametric study on hydrogen storage in AB5 hydride alloys: Heat and mass transfer, thermodynamics, and design

The design of heat exchangers plays an important role in the performance of solid-state hydrogen storage devices. In this study, three different designs of hydrogen storage devices were investigated, incorporating phase change material (PCM), specifically, RT25HC into their walls in various shapes. A mathematical model, validated using previous experimental data, was employed to calculate heat and mass transfer and determine the reaction time period. The calculations were conducted using ANSYS Fluent, with a constant hydrogen supply pressure of 10 bars and a temperature of 295 K. The absorption properties of the storage reactor were investigated through various parameters such as liquid fraction, hydrogen concentration, and natural convection heat transfer coefficients. Results showed that the geometric structure and PCM distribution significantly impacted the performance of the hydrogen storage devices. Design 1, with a centralized PCM distribution, demonstrated moderate heat transfer rates and a longer reaction time. Design 2, featuring a more distributed PCM layout, improved heat transfer rates and reduced hydrogen kinetics time by 40%. Design 3, which optimized the geometric structure with advanced PCM shapes, achieved the highest heat transfer rates and improved hydrogen kinetics time by 50%. This design also enabled the storage of thermal energy at a high density, ranging from 10.2 J/g to approximately 190.5 J/g. The study provides critical information on the design of efficient solid hydrogen storage processes. The findings suggest significant potential for improved performance in industrial applications such as electric vehicles and domestic energy supply. The high efficiency of the large-scale reactor is due to its well-designed system, which effectively converts chemical energy into thermal energy. Optimizing the reactor geometry and PCM distribution is crucial for achieving superior hydrogen storage and retrieval performance.

A Novel Geothermal Wellbore Model Based on the Drift-Flux Approach

Geothermal energy, being a clean energy source, has immense potential, and accurate wellbore modeling is crucial for optimizing the drilling process and ensuring safety. This paper presents a novel geothermal wellbore model based on the drift-flux approach, tested under three different temperature and pressure well conditions. The proposed model integrates the conservation equations of mass, momentum, and energy, incorporating the gas–liquid two-phase flow drift-flux model and heat transfer model. The key features include handling the heat transfer between the formation and the wellbore, addressing the slip relationship between the gas and liquid phases, and accounting for wellbore friction. The nonlinear equations are discretized using the finite difference method, and the highly nonlinear system is solved using the Newton–Raphson method. The numerical simulation, validation, and comparison with existing models demonstrate the enhanced accuracy of this model. In our tests, the model achieved a high accuracy in calculating the bottom-hole pressure and temperature, with mean relative errors (MREs) significantly lower than those of other models. For example, the MREs for the bottom-hole pressure and temperature of the Rongxi area well in Xiongan, calculated by this model, are 1.491% and 1.323%, respectively. These results offer valuable insights for optimizing drilling parameters and ensuring drilling safety. Comparisons indicate that this approach significantly outperforms others in capturing the complex dynamics of geothermal wellbores, making it a superior tool for geothermal energy development.

Modelling of evaporative falling-film heat transfer over a horizontal tube

This article focuses on evaporative falling film heat transfer through numerical simulations. A two-dimensional symmetric liquid film flowing outside a single horizontal tube was simulated using the open-source CFD software OpenFOAM. The Tanasawa model was employed to consider mass transfer at the liquid–vapour interface, and the isoAdvector VOF method was used for interface tracking. The simulations explored the impact of film flow rate, tube surface heat flux, and tube diameter on the liquid film thickness and surface heat transfer coefficient. Additionally, the critical heat flux and operational limits were considered. The results demonstrate the accurate reproduction of evaporative falling film heat transfer performance by the present model. The film thickness exhibits a similar profile to adiabatic and sensible falling film heat transfer but provides smaller values. Similarly, the evaporative heat transfer coefficient exhibits significantly higher values and a gentle peripheral variation. The influence of heat flux on evaporative falling film heat transfer is contingent on the film flow rate; it can either enhance or weaken the process. Notably, a larger tube diameter negatively affects heat transfer, suppressing the impact of heat flux, and also increases the risk of liquid film dryout. For a given film flow rate, the average heat transfer coefficient increases initially, reaches a maximum value, and then decreases as the wall heat flux is increased. The peak heat flux, termed critical heat flux, increases with film flow rate but decreases with tube diameter. An operational limit exists where the heat flux is lower than the critical heat flux and the film flow rate is higher than the critical film flow rate; under these conditions, evaporative falling film heat transfer operates in a fully wetting status.

Viscoelastic flow instabilities for enhanced heat transfer in battery pack cooling

Effective thermal management plays a critical role in ensuring peak performance for heavy-duty electric vehicle battery packs and high-performance electronics. To address the need for effective heat dissipation, liquid coolants are employed, which exhibit higher heat transfer performance compared to traditional air-cooled systems. However, the associated increased viscosity of such liquid coolants and constricted flow path lead to reduced mixing, thereby deteriorating the thermal performance of the system. Here, we explore the potential of utilizing viscoelastic liquid coolants for enhancing heat transfer from the heated walls of a constricted flow path with obstacles, modeling features of battery coolant channels. At low-Reynolds number regime, viscoelastic fluids exhibit instabilities which can disrupt the thermal boundary layer and enhance fluid mixing in the domain, thereby facilitating wall heat transfer. We employed computational fluid dynamics simulations, validated with experimental observations, to explore the effect of the flow geometry, Weissenberg number, solvent viscosity ratio, polymer extensibility, and Reynolds number on thermal performance. Our study indicates that the heat transfer is enhanced with flow instabilities in viscoelastic fluids, which can be generated exclusively in the flow channel with obstacles by increasing the Weissenberg number, reducing the solvent viscosity ratio, increasing the polymer extensibility, and increasing the Reynolds number. The enhancement in the heat transfer can exclusively be attributed to the elastic properties of the fluid in contrast to the inelastic shear-thinning fluids exhibiting no instability. The insights from the study can play an important role in the development of advanced, high-efficiency battery cooling systems, combining surface engineering and synthesis of optimized heat transfer liquids.

Numerical study of liquid metal magnetoconvection and heat transfer in an electrically conductive square duct

Mixed convection of an electrically conductive fluid in a square duct with imposed transverse magnetic field is studied using Large Eddy Simulation (LES) paradigms. The duct walls are electrically conductive, with the wall conductivity parameter cw ranging from 0 to 0.5. The Reynolds number is Re=5602 and the Prandtl number is Pr=0.0238. The focus of the study is on flows at Hartmann numbers Ha⩽125, Richardson numbers Ri⩽10 and two different thermal boundary conditions are considered: four wall uniform heat fluxes and one-sided heating (fixed wall temperature). The results show that the transition from laminar to turbulent flow depends not only on the ratio Ri/Ha, but also on cw and on the local thermal boundary conditions. In the turbulent regime with one-sided heating, the turbulent heat fluxes play an important role in the total heat transfer, in contrast with the typical behaviours of liquid metals. Moreover, the turbulent and thermal structures are highly dependent on the thermal boundary conditions, which completely alter the flow structure. It is also found that at cw≥0.01 the turbulent heat fluxes decrease.

Liquid plug characteristics and heat transfer performance of a silicon-based ultra-thin flat heat pipe at different incline angles

Silicon-based ultra-thin flat heat pipes (UFHPs) that can be integrated with semiconductor devices provide an opportunity for electronic thermal management. To meet the requirements of various operation states, the impacts of gravity on the liquid plug characteristics and heat transfer performance of an UFHP are investigated by a visualization experiment conducted at different incline angles. Condensate flows back through microgrooves under the capillary pressure and gravity under a low heating power (capillary flow stage), and then excess condensate flows back along the edges of the UFHP under a higher heating power (gravitational flow stage). Over a certain heating power (Q ≥ 14 W), liquid plug fluctuations caused by vapor entrainment are observed with periodic dry-out at the evaporator under large incline angles (α ≥ 60°), corresponding to temperature fluctuations. Intense fluctuations of the liquid plug enhance sensible heat transfer, although the liquid plug occupies the condenser area and impedes the vapor flow. Meanwhile, evaporation at the evaporator benefits from corner-film evaporation until a large area of dry-out occurs. Due to the increased gravity and buoyancy, a larger incline angle is conducive to thermal performance enhancement, but this effect becomes restricted as the heating power rises. At a cooling-water temperature of 20 ℃ and an incline angle of 90°, the lowest thermal resistances of the UFHP yield a value of 1.14 ℃/W at 4 W in the steady stage and a value of 1.22 ℃/W at 18 W in the fluctuation stage, respectively. The heat transfer limit of UFHP with overfilling reaches 24 W, compared with the theoretical heat transfer capacity of 8.9 W at an incline angle of 90°. Additionally, a higher cooling-water temperature is beneficial for the thermal performance in the capillary flow stage, but not in the gravitational flow stage and fluctuation stage. This study provides guidance for the operational design of silicon-based UFHP for electronic thermal management.

Structure effect of wall cooling channel on liquid metal heat transfer in aero-engines

In this study, a model of the cooling channel and an experimental system for heat transfer involving liquid metal are studied, to investigate the influence of the wall cooling channel structure of combustor on the heat transfer process. The error between the experimental and simulated outcomes fall is within 3 %, suggesting that the accuracy of the simulation is satisfactory. The simulation results found that the temperature non-uniformity coefficient can be reduced by 13 % and 7 % through adjusting the aspect ratio under the heat flux of 3 MW/m2 and 1.5 MW/m2 respectively, and the wall temperature of gas side can be reduced by 89 K and 47 K respectively. A smaller aspect ratio is favorable for homogenizing the liquid metal temperature distribution. Longer widths, however, can lead to horizontal thermal stratification, which can be unfavorable for heat transfer. With an aspect ratio and rib thickness of about 0.5 and 0.4 mm, respectively, the cooling channel outlet has the lowest non-uniformity coefficient of temperature and enthalpy, which is conducive to the improvement of the wall energy recovery rate. The optimal cooling channel effectively improves the thermal stratification in channel, and provides theoretical guidance for the suitable application of liquid metal to the combustor wall cooling channel.

Falling film hydrodynamics and heat transfer under vapor shearing from various orientations

Vapor shearing is a common issue encountered in the operations of falling film heat exchangers. The vapor stream effect depends on its orientation. This study investigates liquid film hydrodynamics and heat transfer performance under the influence of vapor streams from different orientations. The results indicate that both orientation and velocity of vapor determine the encountering time and position of the films on the tube's two sides. The liquid film thickness uniformity and the liquid column deflection vary significantly depending on the orientation and velocity of the vapor. Zones of accelerated liquid film, climbing liquid film, liquid stagnation, and transition of liquid film flow pattern are observed. The gradient of film thickness along the tube axis and the deflection in time-averaged peripheral film thickness increase as the vapor orientation varies from 0° to 90° and subsequently decrease as the vapor orientation varies from 90° to 180°. Vapor streams have more pronounced effects on time-averaged peripheral film thickness in regions close to the liquid inlet and outlet. Vapor streams result in changes in peripheral heat transfer coefficients toward the downstream side depending on the orientation and velocity of the vapor. The impact of vapor streams on the overall heat transfer coefficient does not directly correlate with the velocity of the vapor when maintaining the same orientation.