Maximum Heat Transfer Coefficient Research Articles

Effect of the surface form of the herringbone aluminum plate in a plate heat exchanger on the boiling heat transfer performance of ammonia

In this study, ammonia boiling heat transfer coefficient and the overall heat transfer were measured in a plate evaporator for an ocean thermal energy conversion (OTEC) system using aluminum plates. The OTEC system used ammonia as the working and a plate heat exchanger (PHE) that combined titanium plates. In this study, we used an anodized aluminum herringbone plate with two chevron angles of 45 and 60° installed in a PHE and measured the boiling heat transfer coefficients of ammonia and overall heat transfer coefficients for both plates to compare their heat transfer performances. The experimental conditions included a hot water flow rate of 1 – 12 L/min, a working fluid mass flux of 7 – 30 kg/m2s, a hot water inlet temperature of 30 – 40 °C, and a system pressure of 700 kPa.It was observed that anodized aluminum can be used as a heat-transfer plate for the PHE. In addition, the maximum overall heat transfer coefficient of chevron angle of 45° was 4 kW/m2K, and that of 60° was 3.5 kW/m2K, respectively. The maximum ammonia boiling heat transfer coefficient of 45° was 12.5 kW/m2K, and that of 60° was 13 kW/m2K, respectively. The heat transfer coefficient values are similar. Additionally, the correlation between the nondimensional heat transfer and the Lockhart-Martinelli parameter of aluminum plates was also clarified for comparison with the previous ammonia plate heat transfer in a flat plate. By comparing the correlation equations, the aluminum plate exhibited a value twice that of the flat plate.

Boiling heat transfer characteristics of distributed jet array impingement on metal foam covers with different wettability

Wettability may have significant influence on jet impingement boiling on metal foam, but the effect mechanism of metal foam wettability remains unclear. In this study, the boiling heat transfer characteristics of distributed jet array impingement on hydrophobic and hydrophilic metal foam covers were experimentally researched and compared with those on uncoated metal foam covers to analyze the influence of wettability. The experimental conditions cover contact angles of 14.0-158.7°, pore densities of 20-40 PPI, porosities of 92%-97%, thicknesses of 3.0-5.0 mm, and jet velocities of 0.5-4.0 m·s−1. The results show that, the obtained maximum heat flux and maximum heat transfer coefficient are up to 538.1 W·cm−2 and 57.9 kW·m−2·K−1, respectively; the hydrophobic metal foam cover has a 4.8 K lower surface superheated degree at the onset of nucleate boiling, but a 7.5% lower maximum heat transfer coefficient compared with the uncoated one; the hydrophilic metal foam cover shows a less deterioration after the departure from nucleate boiling but a 5.3 K higher surface superheated degree at the onset of nucleate boiling than those of the uncoated one. A new correlation for boiling heat transfer coefficients was developed with a mean relative error of 9.75%.

Simulation of the effect of the structure and location of non-closed droplet microchannel on flow/ thermal performance

In order to investigate the effects of geometric parameters and flow modes on the fluid flow morphology, resistance and heat transfer effect in the non-closed droplet pin fin array, in this manuscript, a numerical simulation has been conducted under Re = 100–700 with different pin fin heights (Hp = 0.3 mm, 0.5 mm, 0.7 mm) and pin fin densities (β = 0.046, 0.059, 0.072), with different flow directions under heat flux (10 W/cm2 – 50 W/cm2) studied as well. It is discovered that increasing the height and density of the non-closed droplet pin fin increases the interference with the fluid, resulting in a greater flow resistance phenomenon. However, the enhancement of turbulence not only exacerbates the flow instability in microchannels, but also further promotes the mixing of hot and cold fluids, making the internal temperature distribution more uniform. In which way, the effect of optimizing heat transfer performance achieves. In addition, as q > 20 W/cm2, the overall heat transfer capacity of flow direction B (the tip of the pin fin as upstream fluid direction) is better than that of flow direction A (the cavity of the pin fin upstream as upstream fluid direction). When q = 30 W/cm2, there is a process transferring from single-phase flow to two-phase flow along with the direction of working fluid flow in the microchannel. Under this condition, the local maximum heat transfer coefficient is 15,502.4 W/(m2·K) in flow direction A and 16,094.8 W/(m2·K) in flow direction B. Furthermore, the comprehensive evaluation concludes that the channel flow/thermal performance is the best with Hp = 0.7 mm, β = 0.046.

Augmentation of pool boiling heat transfer using a microstructured aluminum surface fabricated by ultrasonic cavitation modification

Micro/nanostructure is a popular and effective structure for boiling heat transfer enhancement. In this study, ultrasonic cavitation modification is employed firstly to create microstructured aluminum surface for boiling heat transfer. With the impact of ultrasonic cavitation microjet, numerous micro pits and holes are formed on the surface in one step, constituting microstructures on aluminum plate. The saturated and subcooled pool boiling experiments are conducted at atmospheric pressure using ethanol as a working fluid. It is found that boiling hysteresis is apparent on the modified plate (MP) with the porous microstructure layer, which can be eliminated by periodically fluctuating heat flux. Under the stable saturated boiling condition, the maximum heat transfer coefficient is enhanced by 208% and wall superheat is decreased by 17.6 °C compared to the bare aluminum surface. Under subcooled boiling conditions, the boiling hysteresis on MP becomes weaker and the positive effect of microstructures on heat transfer performance is enhanced. The enhanced heat transfer coefficient of MP is mainly attributed to the increase of nucleation site density and further to the facilitation of bubble coalescence and departure, while the increased critical heat flux under subcooled boiling conditions is derived from the delaying occurrence of mushroom-like bubble.

Experimental investigation of a 10 kW-class flat-type loop heat pipe for waste heat recovery

A flat-type 10 kW-class loop heat pipe (LHP) with a box-type wick was designed and developed to handle the higher heat loads in waste heat recovery applications, such as industrial processing and internal combustion engines. Additionally, a numerical model was developed to predict the overall thermal performance of the LHP. The LHP used a stainless steel wick with a pore diameter of 2.0 µm and pure water as the working fluid. The LHP was tested using two types of cooling for the condenser (forced and natural convection), two types of evaporator orientations (horizontal and vertical), and with and without gravity assist (0.3 m and 0 m). For all the tests, the maximum heat load ranged from 5 kW to 10 kW, with the test with a gravity assist of 0.3 m, vertical evaporator orientation, and natural convection performing the best. This test sustained a 10 kW heat load at a steady-state temperature of 182 °C for the evaporator. During the same test, a maximum evaporative heat transfer coefficient of 92,000 W/m2/K at 4.5 kW and a thermal resistance between the evaporator and condenser value of less than 0.007 K/W was achieved. A numerical model was developed to compare the experimental results with the numerical temperature results for the heater block, evaporator, vapor line, condenser, liquid line, and compensation chamber. Overall, the average temperature difference for all components ranged from 7.1 °C to 10.6 °C, with the horizontal orientation without gravity assist and natural convection test predicting the best. The findings demonstrate that LHPs can handle the higher heat loads that are found in waste heat recovery applications for industrial processing and internal combustion engines.

Study on heat transfer characteristics and enhancement mechanism of supercritical carbon dioxide in adaptive channels

Supercritical CO2 power cycles have promising applications in many fields, the heat transfer of supercritical CO2 is critical to the efficiency and safe operation of cycle system. The adaptive channel was previously proposed by our group to improve heat transfer of supercritical CO2, the performance of heat exchanger with adaptive channel was experimentally studied. Whereas the effect of adaptive channel on heat transfer deterioration of supercritical CO2 and the mechanism remain unclear. In this study, the effect of adaptive channel on heat transfer deterioration of supercritical CO2 and the mechanism are analyzed numerically based on a single channel, the effect of structure on heat transfer is analyzed through the maximum wall temperature and average heat transfer coefficient under the same length and heat transfer area. The results reveal adaptive channel is effective at alleviating heat transfer deterioration with no local peak in wall temperature distribution. The channel variation length of adaptive channel need to be increased from 0.4 to 0.8 with the aggravation of heat transfer deterioration in order to eliminate local wall temperature peak. For the heat transfer deterioration conditions, the maximum wall temperature lowers first and then increases as channel variation length increases, there is a length that makes the maximum wall temperature lowest, which is lowered by more than 40 °C compared with straight channel. The average heat transfer coefficient can be increased by 1.1 ∼ 1.5 times as channel variation length increases. For the no heat transfer deterioration conditions, the maximum wall temperature increases about 30 °C whereas average heat transfer coefficient can be increased by 58 %∼76 % with channel variation length increasing. The mechanism of adaptive channel alleviating heat transfer deterioration is as follows: the fluid velocity always presents an inverted U-shaped distribution along the radial direction in adaptive channel, the radial density difference is less and buoyancy effects heat transfer weakly. The turbulent kinetic energy is kept at high, so the heat near the wall is transferred to the main flow timely.

Forced convective rate and pressure drop through a packed annulus: a numerical simulation

Porous materials are used in engineering and industry due to their heat transmission qualities. This study examined forced convection flow through an annular tube packed with sphere balls of various materials and sizes using numerical analysis. The sphere balls were permeable. Ceramic, plastic, and steel with spherical diameters of 3, 5, and 6 and porosity of 0.4 were tested for heat dissipated and fluid flow. Also, test the impact of steel balls of diameter 6mm with 0.6 and 0.8 porosity. The numerical simulation results are used to analyze the forced convection and fluid flow parameters of a three-dimensional annular tube with continuous heat flux in the Reynold number range (5000-14000). Steel balls had an 80% higher heat transfer coefficient than annular tubes without porous mediums. The simulation showed that inserting the porous media increased annular tube pressure loss. The maximum heat transfer coefficient improved by about 80% when the spherical diameter is 3 mm. Also, the result illustrated the heat transfer coefficient of steel balls Increased by about 79%, 69%, and 49%, with 0.4, 0.6, and 0.8 respectively

Experimental analysis of the storage of thermal power by a thermosyphon amalgamated nanophase change material in thermal management systems

Current research examines thermal power storage by thermosyphon amalgamated nanophase change material in thermal management systems. The experiments were analysed with heat inputs ranging from 40 to 90 W using deionized water, ethyl acetate, hydrofluoroether-7100, and n-propylamine as thermal fluids. The copper‑titanium nanocomposite particles at different concentrations (0.5, 0.3, 0.1, 0.08, and 0.05 wt%) are disseminated in a phase change material (methyl cinnamate). Due to its high thermal conductivity, a 0.5 wt% concentrated nanophase change material was used in this study. The heat transfer effectiveness, charging, and retrieving attributes of the nanophase change material are investigated using thermal fluids. The results indicate that hydrofluoroether-7100 has a lower thermal resistance, maximum heat transfer coefficient, and highest heat dissipation rate of about 0.26 K/W, 203.8 W/(m2‧K), and 98.9 %, respectively, at the maximum heat input. The low enthalpy of vaporization and surface tension are the causes of the acquired results. The maximum thermal power storage by the nanophase change material is 7.2 W during charging due to the influence of vaporized hydrofluoroether-7100 at 90 W, which is attributed to the increased vapor mass flow and shorter time required to reach the steady-state temperature of the melted nanophase change material. The overall exergy efficiency is greater in the case of deionized water and is 4.8 % owing to the greater temperature difference and heat loss. Moreover, the minimum possible power savings of approximately 6.2 % are obtained using the developed setup.

Fluid flow and heat transfer characteristics of a microstructured microchannel heat sink under flow boiling condition

A microchannel heat sink (MCHS) is known as a heat dissipation device from the miniaturized electronic components and devices due to its capability to dissipate high heat flux from smaller surface area. However, the higher pressure drop and the uneven heat removal along the channel length are the noticeable disadvantages of MCHS. Though there are attempts to address these shortcomings, no existing study has been found to study the effect of positions of microstructures (micro-pin fin and blind holes) and combined effect of micro-pin fins and blind holes under flow boiling condition. Therefore, in the present study, an attempt has been made to logically explain the effect of various arrangements and positions of pin fins and blind holes in the selected twenty-four uniformly distributed stations on the thermohydraulic performance of MCHS through numerically predicted results. The objectives of the study have been attained using the following three analyses, flow boiling, (i) with saturated fluid inlet conditions (inlet Re = 500), (ii) with different inlet subcooling (20, 40, and 60 °C), and (iii) with different heat flux conditions (100, 150, 200 and 250 W/cm2). From the study, it is observed that the pin fin enhances heat transfer in single-phase flow and delays the phase change by disturbing the flow. The blind holes act as nucleation sites and help phase change heat transfer. It is also observed that the ratio of single-phase and two-phase length and arrangement of microstructures in these two regions predominately dominate the heat transfer and fluid flow characteristics of the microchannel heat sink. The optimum performance (maximum heat transfer coefficient and minimum pressure drop) is achieved when twelve pin fins are placed in the upstream half and twelve blind holes are located at the downstream half of the channel. However, design improvement is possible through more computational and experimental analysis.

Inflatable aerodynamic decelerators for CubeSat reentry and recovery: Surface properties

In this investigation, Direct Simulation Monte Carlo Method is used to compute the surface properties of inflatable aerodynamic decelerators (IAD) applied for the reentry, recovery, and reuse of cubeSats. Surface properties calculation is of fundamental importance for flexible thermal protection system materials selection that withstand the harsh reentry environment. Furthermore, simulations are extremely important to detect possible hot spots at the IAD before design and flight. In this work, three IAD configurations are exposed to a rarefied hypersonic flow with freestream conditions that correspond to those found at reentry conditions at 105 km altitude. All geometries are assumed to be axisymmetric and have a maximum radius of 0.3 m from the center line to the shoulder edge. The main differences between the geometries are the angle between the IAD and cubeSat body and the IAD curvature for geometry 3. According to the results, all IAD geometries are effective in protecting the cubeSat during the reentry phase. It is shown that the IAD geometry has a significant influence on the heat transfer, pressure, and skin friction coefficient distribution over the flexible heat shield. The maximum heat transfer coefficient computed for geometry 1, 2, and 3 is 0.90, 0.97, and 0.84, respectively.

Pool boiling heat transfer characteristics of low-GWP refrigerants in a horizontal tube bundle configuration

Heat transfer enhancement techniques have been adapted on the shell side to improve the overall performance of the flooded evaporators, such as finned tubes. In recent years, it has been demonstrated that the metal foam structure can offer enhanced heat transfer performance under pool boiling conditions. However, there are not much research in the open literature that examine the feasibility of metal foam embedded tubes in horizontal tube bundle configurations. Therefore, this paper proposed a novel metal foam embedded tube (i.e., foam embedded outside the tube) to improve the heat transfer behavior of flooded evaporators. The experiments were performed on a horizontal tube bundle with a staggered arrangement. Moreover, the performance of low global warming potential (GWP) refrigerants (R-1234yf and R-1234ze(E)) is compared against R-134a for both plain tubes and metal foam tubes with porosities of 81%, 75%, and 62%. The results showed that a metal foam tube with a porosity of 62% showed a maximum heat transfer coefficient (HTC) enhancement of 291% compared to the plain tube. As compared with R-134a, the HTCs of R-1234yf and R-1234ze(E) are nearly 10% higher and 5% lower, respectively.

Study on the effect law of jet angle on the cooling of hot-rolled L-beam

The uniformity of cooling affects the organizational properties and product quality of the L-beam. Problems such as bending deformation of the L-beam and excessive residual stresses inside the finished product are mainly caused by uneven cooling. The jet angle is an important cooling parameter in the cooling process of L-beam. In this study, the flow field and cooling law during jet cooling of hot-rolled L-beams were investigated by means of numerical simulation using the jet angle as a variable. The jet angle is −45°∼30° on the lower surface of the short edge of the L-beam due to space limitation, and the jet angle is −45°∼45° on the rest of the parts, and the other process parameters are kept unchanged. The velocity distribution, pressure, water flow, and temperature distribution on the cooling surface and the average heat transfer coefficient were analyzed. The results show that the jet angle has a significant effect on the cooling of each part of the L-beam. With the increase of the jet angle, the asymmetry of water velocity, pressure distribution, water flow distribution, and temperature distribution on the surface of the cooling surface increased significantly. The distribution of heat flux corresponds well to the distribution of temperature. As the jet angle increases the surface average heat transfer coefficient gradually decreases, and the maximum average heat transfer coefficient appears in the vertical jet. Gravity will also have a certain effect on the cooling of the L-beam.

Numerical and experimental investigation on boiling heat transfer and flow attributes for the exterior of smooth tubes and T-fin tubes

Experiments and numerical analyses of saturated pool boiling have been carried out on the outer surfaces of horizontal smooth tubes and T-fin tubes. Using a computational model that closely matches the experimental results, the study examined the dynamics of the bubbles as well as the boiling heat transfer coefficients in both the circumferential and axial orientations of the T-fin tubes. For the first time, comprehensive information on the process of high-performance external tube boiling has been made available. The findings demonstrate that an increase in fin density on the surface of the T-fin tubes leads to an abundance of bubble nuclei, with the escaping bubbles exhibiting both increased size and accelerated velocity. Due to the distinctive configuration of the T-fin tube, the local heat flow is lowest at the root fin and highest at the fin tip, indicating that heat transfer is inhibited in the narrow cavity between the two lateral fins. The optimal fin density, which corresponds to the maximum overall heat transfer coefficient has been identified. This states that the enhanced heat transfer mechanism within the T-fin tube is a symbiotic effect of both the increase in heat transfer area and the subsequent degradation of heat transfer fostered by the bubbling of the tube.

Microfluidic controllable production and morphology-independent phase-change heat transfer of boehmite nanofluids

Enhanced boiling heat transfer through nanofluids represents a significant approach to improving the cooling performance of electronic devices. However, the preparation of nanofluids poses challenges regarding production efficiency, morphology control, and structural stability. In addition, the underlying mechanism of flow boiling heat transfer enhancement of nanofluids, particularly changes in vapor behavior, remains a limited understanding. This study presents a facile and straightforward microfluidic strategy for the controllable synthesis of nanofluids toward efficient heat transfer enhancement. The microfluidic flow synthesis approach effectively enhances mixing efficiency at various flow rates, enabling continuous and high-throughput synthesis of boehmite nanofluids with different morphologies. The synthesized nanofluids exhibit long-term stability (>50 days) at room temperature. Both rod- and sphere-shaped nanofluids under equivalent concentrations can significantly improve critical heat flux (CHF) and maximum heat transfer coefficient (HTC) across various operating parameters, but with similar enhancement characteristics. Analysis of the period of bubble growth and burst, bubble size distributions, number of bubbles, and vapor phase fractions reveals that rod- and sphere-shaped nanofluids could shorten the period, enhance surface wetting, increase nucleation sites, and reduce bubble size efficiently and similarly. Furthermore, the effect of the concentration of nanofluid on phase-change heat transfer performance is systematically explored. 0.01 wt% nanofluid behaves the most remarkable enhancement, CHF and maximum HTC are improved by 24 % and 14.4 %, respectively. These findings not only shed new light on the controllable synthesis of functional nanomaterials but also provide important guidelines for the rational design of nanofluids for efficient two-phase cooling of electronic devices.

Maximizing heat transfer potential with graphene nanofluids along with structured copper surfaces for pool boiling

Increasing the demand and depletion of energy sources confront every technological development with energy conversation and its utilization. Boiling heat transfer is the most applicable heat transfer method in many industries, from electronic chip manufacturing to heavy nuclear power plants. Hence, the augmentation of it is of prime importance for future requirements. The present investigation focused on the augmentation of boiling heat transfer by considering a novel V-pattern staggered copper surface for pool boiling in graphene nanofluid media. The study encompasses a detailed exploration of nanofluid concentrations ranging from 0.1% to 0.40 vol% prepared in equal volumes of dimethylformamide and distilled water. The nanofluid's stability is observed to be quite promising, and the results are presented. The wall superheat was drastically reduced to 48.73%, and the maximum heat transfer coefficient enhancement of 83.47% was achieved for 0.04% concentration of graphene nanofluid with a V-pattern surface compared to a smooth copper surface with water. This is due to the micro-level structured convection patterns promoted by the microstructures. The thermophysical properties of nanofluid and peculiar microstructures enhanced the relative heat transfer coefficient by 63.37%. On the other hand, two popular machine learning models, Random Forest Regression (RFR) and Multi-Layer Perceptron (MLP), are used to predict the boiling heat transfer coefficient. Moreover, it is observed that the RFR model fitted the data well and yielded the lowest mean absolute percentage error within the bounds of computational time.

Characterization of dynamics and heat transfer in electro-spray at elevated flow rates

The advancement of semiconductor and microelectronics technologies has underscored the increasing importance of efficient heat management in electronic devices. To overcome the shortcoming of low flow rates of electro-spray, the present study is motivated by using visualization and heat transfer performance experiments to reveal the morphology evolution and heat transfer characteristics of electro-sprays with high flow rates. The results indicate that the morphology of electro-sprays is closely related to the electric field intensity and the flow rate, and can be classified into six modes: dripping mode, spindle mode, cone jet mode, multi-cone jet mode, simple jet mode, and oscillating jet mode. Furthermore, the formation mechanisms and transition rules of these modes are discussed in detail. In addition, the effects of the flow rate, voltage, and spray morphology on the heat transfer performance of electro-sprays was investigated, and strategies for enhancing heat transfer were subsequently proposes. At a flow rate is 800 ml/h and an applied voltage is 6 KV, the critical heat flux (CHF) can reach 100.8 W/cm2, and the maximum heat transfer coefficient (hmax) can reach 14416.55 W/m2K. This study not only enhances the comprehension of high flow rate electro-spray technology, but serves as a vital reference for the design and application of electro-spray cooling devices.

Pool boiling heat transfer: Thermal performance for alternating and extended operational conditions

Oxide formation on metallic heat transfer surfaces poses a risk parameter in high-power thermal systems maintained by multi-phase cooling. In spite of the significance for practical applications, the potential change in chemistry and morphology on the surfaces exposed to continuous or alternating boiling process plays a vital role in heat transfer efficiency. In this comprehensive study, the effect of heat flux rate, boiling crisis, operation time, surface roughness, and pre-oxidation by the system calibration on oxide layer formation and its inevitable effect on boiling heat transfer (BHT) have been evaluated separately. A sort of experimental scenarios rather representing real operating conditions encountered in phase-change cooling apparatuses were carried out on smooth and rough copper substrates in saturated de-ionized water at atmospheric pressure. In order to practice the scenarios, a series of subsequent critical heat flux (CHF) runs and continuous boiling operations at certain heat flux levels ranging between 15 and 60 W/cm2 were performed within different numbers and sequences as a part of the Design of Experiments (DOE). Each operating scenario was analyzed and compared individually, while questions regarding the effect of the number of CHF runs, heat flux level during long-term operation, surface roughness, pre-oxidation, and rigorous long-term stability operation on boiling heat transfer were examined exhaustively. As a result, CHF and maximum heat transfer coefficient (HTC) of all surfaces exposed to boiling processes considerably enhanced at different rates corresponding to applied scenarios. The rate of heat flux level applied on heating surfaces was not determined to be a significant factor, whereas the HTC and CHF enhanced as the number of subsequent boiling crises (CHF runs) ultimately raised. On the other hand, HTC of the surface non-exposed to CHF runs demonstrated a drastic enhancement by 45% after 24 h of continuous boiling exposure at 45 W/cm2 compared to initial performance with 1.0 W/cm2K. The heat transfer from the surfaces exposed to extended long-term operations showed a final stable thermal behavior, while CHF values doubled from an initial value of approximately ∼73 W/cm² to 144.7 W/cm2 as exposure time increased. X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and scanning electron microscope (SEM) analyses were executed to evaluate the influence of micro-nano-scale oxide particles/layers developed on the heater surfaces and to interpret the changes in heat transfer performance.

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 heat-transfer characteristics of spray cooling for microchannel radiators

Spray cooling is an emerging and promising thermal-management solution with a wide range of applications. However, its application in engineering vehicles remains in the formative stage. In this study, an innovative spray-cooling system for engineering vehicles was proposed. The study, using thermocouples and visualisation methods, focused on how factors such as the spray angle, height, volume flow, nozzle aperture, and arrangement impact the uniformity of the surface temperature and heat-transfer enhancement. Optimal cooling was achieved using a spray angle of 60°, height of 90 mm, and flow rate of 500 l/h. Moreover, a nozzle aperture of 1.6 mm demonstrated superior efficiency in energy use and cooling. Higher temperature differentials (90–140 °C) enhanced the heat transfer but led to a 332.81 % increase in the non-uniformity of the surface temperature. Further, the study compared the spray-cooling efficiency between alumina nanofluids and deionised water. Alumina nanofluids with a 0.7 % mass fraction achieved a maximum heat-transfer coefficient of 450.6 W/(m2·K), 15.48 % higher than that of deionised water. Moreover, based on the experimental results, an empirical formula for enhanced heat transfer using spray cooling in construction machinery was developed, providing a valuable reference for guiding engineering practice. The proposed spray-cooling method, which exhibited a two- to three-time improvement over traditional air-cooled systems, holds considerable promise in the field of engineering vehicle applications.

Effect of sub-cooling on fuel channel behaviour during LOCA for Indian PHWR

During LOCA with ECCS failure, the removal of decay heat from the channel to a moderator plays a significant role in determining the fuel bundle temperatures. An experimental investigation has been carried out to study the effect of change in moderator temperature (65 ℃, 75 ℃, and 85 ℃) on the fuel channel at a steady state of 900 ℃ center pin temperature. The results show that at different moderator temperatures, there is insignificant variation in the temperature of PT, whereas for CT appreciable temperature gradient has been observed. The maximum heat transfer coefficient for CT was attained at 65 ℃ moderator temperature. Even with an increase in moderator temperature, channel integrity is maintained, and the moderator acts as an ultimate heat sink for all three moderator temperatures.