Higher Heat Transfer Coefficient Research Articles

Thermal shock fracture in a cracked strip: Incorporating convective heat transfer between lateral surfaces and ambient environment

This paper presents a comprehensive analysis of transient temperature and thermal stress around a thermal insulated crack in a finite thermoelastic strip subjected to thermal shock based on the dual-phase-lag (DPL) heat conduction. A detailed investigation of the effect of convective heat exchange between the lateral surfaces of the strip and the ambient environment is conducted. By employing integral transform technique, the thermoelastic crack problem is reduced to a set of singular integral equations, and solutions for the transient temperature distribution and dynamic stress intensity factors (SIFs) at the crack tips are then obtained. Numerical results reveal a considerable temperature gradient near the heating surface when both the high convective heat transfer coefficient and thin strip are involved. Consequently, the SIFs in such scenarios where cracks are located close to the heating surface are significantly larger compared to those calculated by neglecting the convective heat loss. Furthermore, during the initial period following a thermal shock, the SIFs may exhibit higher magnitudes than the steady-state values. These findings emphasize the importance of accounting for the influence of convective heat exchange with the ambient environment in material design and optimization to enhance fracture resistance under transient thermal loading.

Cooling enhancement for engine parts using jet impingement

A computational study has been performed to evaluate the use of jet impingement for cooling applications in the automotive industry. The current study uses an entire internal combustion engine cylinder with its components as a computational domain. An unsteady numerical solution for the Navier-Stokes equations was carried out using Improved Delayed Detached Eddy Simulation (IDDES). The volume of fluid approach is proposed to track and locate the liquid jet surface that is in contact with the air. The conjugate heat transfer approach is used to link the heat transfer solution between the fluid and the solid. The boundary conditions that are employed in the study are provided from lab experiments and one-dimensional simulations. The cooling jet in this study targets the hottest region in the piston, i.e., the region underneath the exhaust valve. Three nozzle sizes with flows at different Reynolds numbers are chosen to examine the thermal characteristics of the cooling jet. The computational study reveals that for a specific Reynolds number, the smaller diameter nozzle provides the highest heat transfer coefficient around the impingement point. The maximum relative velocity location at the impingement point slightly leads the location of the maximum Nusselt number. The maximum temperature in the piston decreases by 7% to 11% as the nozzle diameter changes from 1.0 to 3.0 mm for a jet Reynolds number of 4,500. If a correct selection is made for the nozzle size, the cooling jet can be efficiently used to reduce the temperature and alleviate the thermal stresses in the piston in the region underneath the exhaust valve where the maximum temperature occurs.

Highly efficient pool boiling heat transfer on surfaces with zoned rose-petal-inspired hierarchical structures

Efficient boiling heat transfer depends on the rapid formation and detachment of vapor bubbles on solid surfaces. However, this process is hindered by the contradictory requirements of bubble nucleation and departure. Herein, a novel surface with zoned rose-petal-inspired hierarchical structures is prepared on copper by laser-based methods to promote bubble nucleation and detachment simultaneously. The surface features parahydrophobic rose zones surrounded by superwetting pump zones. The rose-petal-inspired hierarchical structures on the rose zones greatly enhance the bubble nucleation, resulting in very high heat transfer coefficients (HTC). The pump zones with hierarchical mini-nanostructures facilitate the formation of distinct pathways for liquid and vapor flows, enabling efficient liquid rewetting and bubble departure at high heat fluxes. The synergistic effects of the rose and pump zones enhance both bubble nucleation and departure, leading to superior boiling performance. The critical heat flux (CHF) and the HTC achieved at the CHF (HTC@CHF) increase by 100% and 360%, respectively, compared to a smooth surface. Our results provide new avenues for the development of metal-based highly efficient boiling surfaces, which may enhance the efficiency of energy utilization and heat dissipation of various thermal management devices.

Heat transfer characteristics of R134a flow boiling in a microfin tube under typical ORC pressures based on comparison with a smooth tube

The experimental investigation on the flow boiling heat transfer of an organic working fluid under typical organic Rankine cycle (ORC) operating conditions is insufficient, which directly affects the performance of the ORC system. In this paper, the flow boiling characteristics of R134a in a horizontal microfin tube were experimentally investigated in the reduced pressure range 0.6 ∼ 0.9. The general convection heat transfer characteristics of R134a flow boiling, along with the effects of heat flux, mass flux, pressure, and rewetting conditions, were investigated in a microfin tube and compared with that in a smooth tube. Results show that the main flow regime in the microfin tube under typical ORC operating pressures is the stratified flow with dry-out occurring at the top surface. The microfin effectively enhances the heat transfer and restrains the wall temperature separation through inducing strong helical flow due to buoyancy and centrifugal forces. With the heat flux increasing, the micro-fin strengthens the nucleate boiling by enhancing the vaporization cores, leading to more pronounced heat transfer enhancement compared with that in the smooth tube. The heat transfer on the bottom surface shows almost negligible sensitivity to variations in mass flux. However, in regions of high vapor quality, an increase in mass flux results in higher heat transfer coefficients on the top surface. Due to the interaction between intensified nucleate boiling and weakened gas convection when pressure is increased, it slightly affects the heat transfer coefficient. Unlike the smooth tube, the rewetting in the microfin tube can be maintained within a larger range of vapor quality due to the spiral flow caused by the micro-fin.

Numerical Heat Transfer Analysis of Evacuated Tube Solar Collector Using MXene/Soyabean Oil Nanofluid

This study focuses on investigating the thermo-hydraulic performance and entropy generation rate of an evacuated tube solar collector using MXene/Soybean oil as a working nanofluid by means of Computational Fluid Dynamics (CFD). A complete 3-D geometry is considered using three different kinds of working fluids (water, soybean oil and MXene/Soybean oil-0.025 wt.%, 0.075 wt.%, and 0.125 wt.%) for five different mass flow rates (0.02475 kg/s, 0.04818 kg/s, 0.0717 kg/s, 0.0952 kg/s, and 0.1188 kg/s) under same solar radiation (925W/m2). The results revealed that MXene/SO-0.125 wt.% at 0.0717 kg/s showed the best thermo-hydraulic performance because it achieves the balance between a high heat transfer coefficient and a low-pressure drop. However, MXene/SO-0.125 wt.% has been shown to have the highest entropy generation rate compared to other loadings because thermal efficiency is directly proportionate to the entropy generation rate. Therefore, the final recommendation for the best working fluid is MXene/SO 0.075 wt.% at 0.4818 kg/s because it attains good thermal efficiency, low entropy generation rate, and when compared to water has shown the highest improvement of 31.1%.

ENHANCEMENT EVALUATION CRITERIA FOR POOL BOILING ENHANCEMENT STRUCTURES IN ELECTRONICS COOLING: CHF ENHANCEMENT RATIO (ER-CHF) AND ENHANCEMENT INDEX (EI)

Extensive research shows the necessity of efficient cooling systems to enable electronic components to operate at high performance levels for a sustained period. While conventional methods have served the cooling needs so far, rising computational power, energy efficiency, and sustainability requirements call for improved techniques. The literature shows the effectiveness of two-phase systems in cooling electronic components like microprocessors. The literature further describes various enhancement mechanisms to elevate the critical heat flux (CHF) and heat transfer coefficient (HTC) in these systems. While a high CHF is desired, having a high HTC is equally important to keep the operating temperatures below a permissible limit. The present article summarizes enhancement structures found in the literature suitable for electronic cooling to provide this dual enhancement in CHF and HTC. New enhancement evaluation criteria that also consider the surface temperature limit imposed by the electronic components are introduced. The CHF enhancement ratio (ER<sub>CHF</sub>) represents the ratio of CHF for enhancement structures to the CHF for a plain surface, and the enhancement index (EI) represents the ratio of wall superheat at CHF with the enhanced structures to the wall superheat at its respective CHF condition for a plain surface. It is desirable to have a high value of ER<sub>CHF</sub> coupled with a low value of EI (lower the better), preferably below 1.0.

A high heat dissipation strategy based on a multi-scale porous hydrogel and heat sink exhibiting cooling capacity comparable to that of forced air convection but with zero energy consumption

A new composite heat sink (CHS) is designed based on a multi-scale porous hydrogel. Owing to the high heat transfer coefficient and zero energy consumption property, the CHS shows great potential in electronic device cooling field.

Investigation of water phase change rotating cooling for high temperature turbine

Thermal protection of high temperature turbine restricts the performance of gas turbine. In order to increase turbine inlet temperature, water phase change rotating cooling scheme for turbine was proposed for using the gasification latent heat to improve cooling capacity. Numerical model of phase change rotating cooling was established to study the flow and heat transfer characteristics with Ansys CFX software using SST k-ω turbulence model. Results showed that phase change rotating cooling had good performance for cooling the rotating blades. The temperature drops of blades were more than 400 K with the average heat flux of 2.42 MW/m2. Due to the influence of flow thermal acceleration effect, the outlet pressure of cooling channel reached a maximum value of 7.23 MPa with a mass flow rate of 0.17 g/s. The outlet quality distribution under rotating condition was relatively uniform than nonrotating condition. The temperature increased firstly and then decreased in centripetal channel along phase change process because of pressure variation. Meanwhile, gas-liquid interface disappeared due to the Coriolis force, resulting in higher heat transfer coefficient than static condition. The normalized Nusselt number (Nus) of phase change rotating cooling reached 3.915, much higher than static liquid cooling.

Effect of RIBS/FINS and Aspect Ratio on Flow Boiling Characteristics in Conventional Channels

Abstract In this work, experiments are conducted with conventional rectangular channels of two different aspect ratios (AR = w/d) for the horizontal boiling flow conditions at atmospheric pressure. Distilled water was used as the working substance. The heat transfer coefficients (HTC) were measured for mass fluxes and heat fluxes ranging from 85.94 kg/m2-s to 343.77 kg/m2-s and 10 kW/m2 to 100 kW/m2, respectively, and at inlet subcooled temperatures of 303 K, 313 K, and 323 K. Visualization of the boiling phenomenon was done using a high-speed camera for the two channels under similar conditions. The results show that the AR has a dominant effect on the HTC. At low heat flux values, higher HTC was noticed for the channel of higher AR (AR = 1.25) whereas, at high heat flux conditions, the HTC is higher for the channel of lower AR (AR = 0.2). With an increase in inlet subcooled temperature, the HTC decreased for both channels due to increased thermal boundary layer thickness and reduced bubble formation. Further, the channel of AR = 1.25 with ribs/fins performed better than the smooth channel due to the high bubble nucleation rate.

Unsteady numerical study of film cooling and heat transfer on turbine blade squealer tip with coolant jet

Cooling performance of blade squealer tip with film holes in its cavity was investigated numerically under unsteady condition, and the effects of blowing ratio (BR), cavity depth, and tip clearance were comprehensively discussed. Results show that flow inside tip gap periodically fluctuates and affects the outlet pressure, BR, and jet characteristics of film hole, resulting in unsteady tip cooling effectiveness which could not be observed in steady condition. Average cooling effectiveness on 40-70% axial chord of tip changes the most over time. In comparison to steady result, unsteady time-average cooling effectiveness of tip and its region before 40% axial chord is lower while it is relatively higher in the region after 40% axial chord. Coolant from tip hole located in the flow area of rolling vortex is strongly mixed with rolling vortex, leading to high heat transfer coefficient downstream of that hole. Unsteady leakage flow and boundary layer flow cause stronger heat transfer on squealer rim than steady result. Small BR under unsteady condition could result in insufficient coolant supply and high temperature gas intrusion in tip holes located in 40-70% axial chord of tip. Increasing cavity depth brings longer normal jet distance and higher momentum of coolant, leading to coverage deterioration on floor with a drop in tip average cooling effectiveness of 11.6-34.7%. Large tip clearance causes the distribution of BR to become more uneven, and then more coolant flows out from trailing region, resulting in decrease tendency of average cooling efficiency with change in the range of −23.4%∼6.1% over time compared to base design.

A comparative study of the impact on combustion and emission characteristics of nanoparticle‐based fuel additives in the internal combustion engine

AbstractThe search for an effective solution to improve performance and emission characteristics of internal combustion (IC) engines used in the commercial sector is regarded as one of the most important and essential issues in recent years due to increasing levels of pollution. Nanoparticles with their additive ability to increase fuel reactivity and atomization, due to their large surface area and high heat transfer coefficient, can improve the performance and emission characteristics of a fuel. This review highlights the use of nanoparticles as fuel additives to enhance the emission and performance characteristics of IC engines. Detailed comparisons of performance, emission, and combustion characteristics of IC engines using fuels blended with nanoparticles have been done. Nanoparticles were observed to be an oxygen buffer for fuel combustion and boost fuel atomization, thus enhancing engine performance. While alumina exhibited a decrease in levels of HC and CO but a considerable increase in NOx, graphene nanoparticles and ceria were found to be particularly effective in enhancing engine performance. Detailed study has been done on other nanoparticles, including metal‐oxide, nonmetal‐oxide as well as carbon nanoparticles. Overall, the use of nanoparticles can enhance the thermophysical characteristics of fuels, improving the emission and performance characteristics of engines. The review suggests that selecting the right dosage and variety of nanoparticles is crucial for optimizing engine performance, and thus directly helps in tackling the ongoing pollution problem.

Experimental investigation of the convective heat transfer coefficients at different angular orientations inside the void in a cold flow pebble bed reactor using a non-invasive heat transfer pebble probe

This study focuses on investigating the effect of the surface location of the pebble inside the bed, by varying the angular orientation of the heat transfer pebble probe and the velocity of the gas on the convective heat transfer coefficients in a cold-flow pebble bed reactor. An advanced non-invasive technique consisting of a cartridge heated copper pebble, a micro-foil sensor and a thermocouple probe was used to obtain accurate measurements of the local heat transfer coefficients from three surface locations inside the void. The effect of the wall on the local convective heat transfer coefficients was highlighted due to the low aspect ratio of the bed (bed diameter to pebble diameter (5cm)) used in this work. The local measurements showed, for the first time ever, that the surface location inside the void substantially impacts the heat transfer coefficients, with higher heat transfer coefficients at the center of the void and bottom of the void. Furthermore, the measured overall heat transfer coefficients showed good agreements with the predictions of the correlations of [52,21] and [14]. A second order polynomial model with an R2=0.9807 and an average absolute relative error (AARE) equal to 9.08%, was developed for the prediction of the local heat transfer coefficient within the design and operation conditions of this work. The accurate local heat transfer data collected in this work is valuable as benchmark data for the validation of computational fluid dynamics (CFD) simulations and heat transfer calculations.

Numerical study of falling film flow and heat transfer on the horizontal tube with a porous coating layer

Porous surfaces often exhibit exceptional performance in liquid spreading and heat transfer, making them promising candidates for enhancing falling film heat transfer on solid surfaces. This paper delves into the flow and heat transfer characteristics occurring within falling films on horizontal tubes coated with a porous layer. To achieve this, a three-dimensional numerical model that couples the Volume of Fluid (VOF) method with porous media considerations was employed. The investigation comprehensively covers film spreading behavior, flow patterns, film velocity, film thickness, temperature distribution, and heat transfer coefficients, for both plain and porous tubes. Furthermore, we delve into the effects of key parameters, such as porosity, layer thickness, and material properties, on the overall heat transfer process. The results demonstrate the accuracy of our model in reproducing film thickness and heat transfer coefficients within falling films on horizontal porous tubes. When comparing plain tubes to porous ones, some key differences emerge. Specifically, the liquid film on porous tubes spreads more slowly, leading to increased film thickness and fewer occurrences of dry patches. Additionally, porous tubes result in lower film velocity and a more uniform temperature field compared to plain tubes. Interestingly, the findings reveal that smaller porosities yield thicker films and higher heat transfer coefficients, particularly when the porosity is below 0.4. Moreover, thicker porous layers result in thinner films, higher wetting ratios, and increased heat transfer coefficients. Among the materials we tested, aluminum and copper porous layers exhibit the highest heat transfer performance, while steel demonstrates the lowest.

Pool Boiling of Ethanol on Copper Surfaces with Rectangular Microchannels

In this paper, pool boiling of ethanol at atmospheric pressure was analyzed. The enhanced surfaces were made of copper, on which grooves with a depth ranging from 0.2 to 0.5 mm were milled in parallel. The widths of the microchannels and the distances between them were 0.2 mm, 0.3 mm and 0.4 mm, respectively. The highest heat transfer coefficient, 90.3 kW/m2K, was obtained for the surface with a microchannel depth of 0.5 mm and a width of 0.2 mm. The maximum heat flux was 1035 kW/m2. For the analyzed surfaces, the maximum heat flux increase of two and a half times was obtained, while the heat transfer coefficient increased three-fold in relation to the smooth surface. In the given range of heat flux 21.2–1035 kW/m2, the impact of geometric parameters on the heat transfer process was presented. The diameters of the departing bubbles were determined experimentally with the use of a high-speed camera. A simplified model was proposed to determine the diameter of the departure bubble for the studied surfaces.

Optimized Design of Shell and Tube Heat Exchanger with Segmental Baffles

Heat exchangers are the apparatus which is widely used in various industries. They transmit heat among two or more fluid streams. Theoretical analysis done by Kern’s method has been done on Shell and tube heat exchanger with segmental baffle and then experimented in CFD to check Heat Transfer rate and pressure drop with varying number of baffles which is 6, 8 and 10, and by keeping all the rest of parameters constant. It is found that as the number of baffles increase heat transfer rate increases also pressure drop is increases significantly. To optimize model in next step, CFD analysis is has been done by varying baffle cut to 25%, 35% and 45% by keeping same number of baffles which found best in heat transfer rate in earlier Kern’s method. Other parameters like velocity, temperature, pressure and baffle numbers are kept constant. Results obtained from numerical solution are analysed extensively to get the effect of baffle cut on heat transfer rate and pressure drop on shell side. The pressing need of the heat exchanger industry, the tradeoff optimization between the pressure drop and the heat transfer coefficient has been studied to provide an idea to get effects of the change in two parameters, namely, baffle spacing and baffle cut simultaneously and observe how these two will affect the performance of an STHX. An attempt to find the optimum geometric configuration has been carried out with number of baffles. Increasing baffles for same length of shell and baffle cut increases from 25% to 35%, heat transfer rate increases. It is beneficial in elucidating that a higher heat transfer coefficient can be obtained for fewer baffles when coupled with the appropriate baffle cut. So selection of correct optimized shell and tube heat exchanger. The work is carried out for copper tube bundles and steel shell. So optimization of baffle cut with correct number of baffles and reduction in pressure drop studied with CFD tool.

Experimental study of R448A flow boiling in a 24-port microchannel tube

R448A is a mixture to replace R404A in the thermodynamic cycle applications. R448A has a temperature glide of about 5°C and might exhibit thermal non-equilibrium in the microchannel tube. This study investigated the flow boiling heat transfer and pressure drop of R448A, a five-component non-azeotropic mixture, in a microchannel tube. Six heat transfer coefficients (HTC) and pressure gradients (dP/dz) of R448A were measured in one pass in a 24-port microchannel tube. The experimental conditions covered mass fluxes from 100 to 200 kg-m−2s−1, heat fluxes from 2 to 8 kW- m−2, and saturation temperatures from 10 to 30 °C. The study compared the experimental results of R448A with four of its component fluids (R134a, R1234yf, R1234ze(E), and R32). R448A had higher HTC and lower dP/dz than its component fluids, except for R32 which had higher HTC and dP/dz than R448A. Muller-Steinhagen and Heck was the best correlation for predicting the dP/dz of R448A, with a mean absolute error (MAE) of 8.78 % and a mean error (ME) of 7.47 %. A new correlation for predicting the HTC of R448A, based on modifying Liu and Winterton (Müller-Steinhagen and Heck, 1986), was proposed with an MAE of 6.01 % and an ME of 0.588 %.

Response surface methodology-based multi-nozzle optimization for electrospray cooling

The performance of electronic systems has decreased due to an increase in heat flux density. New methods such as electrospray cooling are promising to ensure that these systems operate under desired conditions. In electrospray cooling, the working flow rate is limited because the small nozzle diameter is preferred to improve the electric field strength and heat transfer performance. However, it is known that increasing the fluid flow rate is a good method to improve heat transfer. For this reason, multi-nozzles should be used to remove more heat from the surface with electrospray cooling. For the first time in the literature, multi-nozzles were optimized for electrospray cooling with the optimization method in the study conducted for this purpose. Nozzle optimization studies were carried out for ethanol–water mixture used as a fluid for the first time in the literature, nozzle diameter, distance between nozzles, and nozzles number. In the experimental study, it was determined that the fluid composition ratio was effective on electrospray cooling and the effects of nozzle geometry parameters (nozzle diameter, distance between nozzles, nozzle number) were limited. As a result of the optimization process based on the response surface methodology, the highest heat transfer coefficient was obtained for the 5 mm nozzle-center distance where 3 pieces of 26G nozzles.

Effects of shark bionic V-groove structures and nanofluids on thermal management of electronic components

The miniaturization of electronic components makes it difficult for the traditional radiator working medium and radiator structure to meet the requirements. The combination of nanofluids and bionic micro-surfaces is beneficial to solve the heat dissipation problem of electronic components. Based on the bionics, a CPU enhanced heat dissipation system was designed and built based on the V-shaped groove of shark scales and Fe3O4-H2O nanofluids with high heat transfer coefficient. The effects of V-groove angle, height and nanofluids concentration on the comprehensive evaluation index of the radiator were studied. The groove angle S= 1 mm (54°) and height H= 2 mm have the best heat transfer characteristics and comprehensive evaluation index, which can reduce the surface temperature by 2.16 °C and 3.71 °C at most, and increase the heat transfer efficiency by 17.58% and 28.96%. There is a critical concentration (0.3%) that makes the system cooling effect best. The results of the study have a certain reference value for the structural design of heat sink

Review and Analysis of Heat Transfer in Spacer-Filled Channels of Membrane Distillation Systems.

Membrane distillation (MD) is an attractive process for the concentration of seawater brines. Modelling and simulation of membrane distillation processes requires a better knowledge of the heat transfer coefficients in spacer-filled channels which are usually determined by applying empirical correlations for the Nusselt number. In this study, first, a comprehensive literature review on heat transfer correlations was conducted. It was found that the empirical correlations often used for MD simulation result in strongly varying Nusselt numbers that differ by up to an order of magnitude at low Reynolds numbers. Then, heat transfer in spacer-filled channels was investigated experimentally in a membrane distillation system using an aluminum plate instead of a flat-sheet membrane. Numerous tests were carried out with sodium chloride solutions in a wide range of salinities, between 1 g/kg and 95 g/kg, and temperatures, between 30 °C and 80 °C, yielding high heat transfer coefficients in a range of 1500 to 8300 W/(m2K) at relatively low Reynolds numbers, between 100 and 1500, clearly showing the influence of the spacers on heat transfer. A new empirical Nusselt correlation (Nu=0.158Re0.652Pr0.277) was derived which represents the experimental data with a deviation of 10% and is valid for 100<Re<1500 and 2<Pr<7. Computational fluid dynamics simulations were performed to analyze the variations of the fluid properties across the boundary layer due to temperature differences. The simulations showed only minor deviations of the heat transfer coefficients in the hot and cold fluid channels for small driving temperature differences.

Water, ethanol, or methanol for adsorption chillers? Model-based performance prediction from Infrared-Large-Temperature-Jump experiments

Adsorption chillers are a promising technology to ease the burden on renewable electricity demand by using waste heat as driving energy. Their performance depends on the adsorbent’s kinetics, the chosen refrigerant, the design of the adsorber, and the selected temperatures. Thus, quantifying an adsorption chillers’ performance usually requires expensive experiments at full scale. Here, we efficiently characterize a broad set of working pairs for a wide range of conditions using a two-step approach: First, we conducted Large-Temperature-Jump and equilibrium experiments to extract kinetic and equilibrium parameters of water, ethanol, and methanol with a set of currently discussed adsorbents. Second, we inserted the parameters into a dynamic Modelica model to evaluate the working pairs’ performance in full-scale two-bed adsorption chillers. For chilling above 0 °C, water remains the benchmark refrigerant with the highest volumetric power and efficiency with the silica gels SG123 and Siogel. Ethanol and methanol working pairs offer higher mass transfer coefficients, but could not offset the benefits of water. Coatings of the MOFs aluminum fumarate and CAU–10–H provided exceptionally high heat transfer coefficients, but were too thin to compete. For chilling below 0 °C, ethanol and methanol were on par. ZIF–8 was competitive with the activated carbon CarboTech A35. Overall, CarboTech A35/methanol showed broad applicability, performing comparably to SG123/water above 0 °C while still providing reasonable performance below 0 °C.