Convective Heat Transfer Coefficient Research Articles

A computational approach to analyze apple dehydration using hybrid active solar dryer

AbstractIn the present research, the control parameters that is, bed area, water flow rate, and No. of air exchanges of Hybrid Active Solar Dryer integrated with Evacuated Tube Collector located at the solar energy laboratory of Madhav Institute of Technology and Science (MITS) Gwalior (M.P.) India, have been optimized. Moisture removal (mev), convective heat transfer coefficient (hc), evaporative heat transfer coefficient (he) and drying rate (Rd) have been taken as response parameters. Experiments have been conducted in different seasons according to Taguchi's L9 orthogonal array (OA). Response data has been analyzed independently and simultaneously. With the help of signal‐to‐noise (S/N) ratio, optimal parameter setting has been determined for good performance in different seasons. Entropy measuring techniques are used to determine the entropy weightage of each response. Gray relational analysis integrates several responses into a single one, that is, gray relational grade (GRG). The value of predicted S/N ratio for GRG as per optimal parameter setting is −0.04706 which is higher than the experimental S/N ratio which is −0.48614, indicating the successful implementation of this approach.

Multi-objective geometric optimization of protrusion, droplet ribs, and inclined upper plate of a slit jet impingement heat sink to enhance its thermal performance

To meet the needs of heat transfer and temperature uniformity in the heat dissipation of high-heat-flux electronic devices, a novel slit jet impingement heat sink with a protrusion in the stagnation region, droplet ribs in the wall jet region, and an inclined upper-plate (PDI-SJIHS) is proposed, the coolant is Al2O3-H2O nanofluid. The influences of the structural parameters of the above three components and the Reynolds number on the heat transfer and flow of the PDI-SJIHS are studied numerically. Using the comprehensive heat transfer and temperature standard deviation as objective functions, a multi-objective optimization of PDI-SJIHS is conducted using non-dominated sorting genetic algorithm-Ⅱ (NSGA-Ⅱ). The results indicate that the combination of the three components enhances the thermal performance of the PDI-SJIHS, compared with the slit jet impingement heat sink with only one of the above components. Rising the Reynolds number can enhance the thermal characteristics of the PDI-SJIHS. For a Reynolds number of 8000 and nanofluids with a 3% volume fraction, the average convective heat transfer coefficient, friction coefficient, and comprehensive heat transfer coefficient of the optimized PDI-SJIHS are increased by 182%, 153%, and 108%, respectively, and the standard deviation of temperature is 91% less than that of F-SJIHS.

Study on the thermal control performance of lightweight minimal surface lattice structures for aerospace applications

With the rapid evolution of aerospace technology and the increasing complexity of space environments, the demand for lightweight, thermally insulated, and highly efficient heat dissipation materials in aircraft equipment has surged. This study addresses this critical need by investigating Triply Periodic Minimal Surface (TPMS) lattice structures, offering a novel design approach to enhance spacecraft thermal management under extreme temperatures. Our work introduces a methodology to quantitatively assess the thermal insulation and heat dissipation performance of various lightweight lattice sandwich structures. We constructed implicit function models of low volume fractions, including Gyroid, Diamond, Primitive, and IWP, and conducted experiments using an active cooling setup under controlled heating conditions at 300 °C. Key findings reveal that, at flow velocities ranging from 0.25 to 1.5 m/s, the Diamond model exhibited the highest convective heat transfer coefficient, surpassing the Gyroid, Primitive, and IWP models by 6.9 % to 427.3 %, 87.1 % to 485.6 %, and 1.5 % to 98.7 %, respectively. Conversely, at velocities between 1.5 and 3.75 m/s, the Gyroid model demonstrated superior heat exchange performance, improving by 14.6 % to 67.2 %, 73 % to 167.7 %, and 9 % to 64.8 % compared to the Diamond, Primitive, and IWP models. During thermal insulation tests at temperatures from 200 to 400 °C, the Diamond model showed the best insulation effect, while the Primitive model performed poorly. Based on the experimental data, we established empirical relationships between the Nusselt number, friction coefficient, and Geometric Surface Ratio (GS). These findings not only underscore the significant potential of TPMS lattice structures in aerospace applications but also provide a robust theoretical framework and practical guidelines for achieving efficient thermal management in lightweight aerospace manufacturing.

Local characteristics and predictive models of the natural convective heat transfer coefficient for human body in standing and sitting postures

Local characteristics and predictive models of the natural convective heat transfer coefficient for human body in standing and sitting postures

Heat and Mass Transfer in Shrimp Hot-Air Drying: Experimental Evaluation and Numerical Simulation

Shrimp is one of the most popular and widely consumed seafood products worldwide. It is highly perishable due to its high moisture content. Thus, dehydration is commonly used to extend its shelf life, mostly via air drying, leading to a temperature increase, moisture removal, and matrix shrinkage. In this study, a mathematical model was developed to describe the changes in moisture and temperature distribution in shrimp during hot-air drying. The model considered the heat and mass transfer in an irregular-shaped computational domain and was solved using the finite element method. Convective heat and mass transfer coefficients (57.0–62.9 W/m2∙K and 0.007–0.008 m/s, respectively) and the moisture effective diffusion coefficient (6.5 × 10−10–8.5 × 10−10 m2/s) were determined experimentally and numerically. The shrimp temperature and moisture numerical solution were validated using a cabinet dryer with a forced air circulation at 60 and 70 °C. The model predictions demonstrated close agreement with the experimental data (R2≥ 0.95 for all conditions) and revealed three distinct drying stages: initial warming up, constant drying rate, and falling drying rate at the end. Initially, the shrimp temperature increased from 25 °C to around 46 °C and 53 °C for the process at 60 °C and 70 °C. Thus, it presented a constant drying rate, around 0.04 kg/kg min at 60 °C and 0.05 kg/kg min at 70 °C. During this stage, the process is controlled by the heat transferred from the surroundings. Subsequently, the internal resistance to mass transfer becomes the dominant factor, leading to a decrease in the drying rate and an increase in temperatures. A numerical analysis indicated that considering the irregular shape of the shrimp provides more realistic moisture and temperature profiles compared to the simplified finite cylinder geometry. Furthermore, a sensitivity analysis was performed using the validated model to assess the impact of the mass and heat transfer parameters and relative humidity inside the cavity on the drying process. The proposed model accurately described the drying, allowing the further evaluation of the quality and safety aspects and optimizing the process.

Effect of nozzle arrangement on heat dissipation performance and mechanical efficiency loss of bionic texture gear

Abstract Friction and wear are key issues in gear meshing. Lubrication, therefore, is used to reduce heat. The position and direction of the flow of the lubrication determined by the way the nozzles are injecting the fluid and by the texture on the flanks of the teeth, both have an influence on the heat transfer. Furthermore, the geometric characteristics of the tooth texture influences the friction behaviour directly. This article presents investigations on the impact of the tooth texture on wear. A proposal to prepare the tooth flanks with arc shaped grooves is made presenting the manufacturing process. Different measurements to determine the tooth flank texture and the friction are performed. Measurements showed that the arc groove gear had lower friction coefficients, wear depth, surface roughness Ra, and maximum tooth profile peak height Rq compared to conventional gears. The dynamic process of gear meshing is simulated by FEM analysis to understand the physics of heat flow and friction in detail，which revealed that the arc groove gear had reduced sliding distance, contact pressure, and wear depth, along with a higher convective heat transfer coefficient. Based on the measurement results of orthogonal test, two different nozzle arrangements are predicted: one is linear regression prediction (nozzle arrangement A), and the other is nonlinear particle swarm optimization neural network prediction (nozzle arrangement B). By measuring the torque loss value under the two nozzle arrangement conditions, it is found that the nozzle arrangement B is the best arrangement. The specific parameters are: injection distance is 56.0674 mm, injection angle is 8.4527°, nozzle diameter is 3.3955 mm, and pinion speed is 3000r / min. Under this condition, the mechanical efficiency loss is reduced by 94.48 %.

Performance analysis of superconducting motors for electrical aircraft propulsion using electromagnetic–thermal coupling method

The aviation industry is focusing on the development of low-carbon emission technologies, with hydrogen-powered aviation hybrid technology being an important area of interest. Superconducting motors (SCMs) play a crucial role in these aviation hybrid systems due to their high power-to-weight (PTW) ratio. However, SCMs face challenges such as strong coupling of the electric field, magnetic field, and temperature field within the motor. Conventional electromagnetic–thermal coupling analysis methods are not suitable for SCMs, as they are slow to perform calculations that are also less accurate. This study proposes an electromagnetic-thermal coupling analysis model for SCMs, taking into account the constraints of critical current, critical magnetic field, and critical temperature of superconducting (SC) materials. The study analyzes the impact of the critical current of SC tapes, permanent magnet remanence, convection heat transfer coefficient between SC coils and cooling medium, AC loss reduction of SC coils, and motor speed on the ultimate power of SCMs. The results show that an optimal permanent magnet remanence can maximize the ultimate power, and the other parameters have been found to be positively correlated with the ultimate power of the SCM. To enhance the PTW ratio of SCMs, future research directions include increasing the critical current of SC materials, reducing the AC loss of SC coils, and improving the cooling effectiveness of SC coils.

Accuracy in evaluating convective heat transfer coefficient by RANS CFD simulations in a rectangular channel with high aspect ratio and V-shaped ribs

Abstract In the framework of heat transfer enhancement inside channels with ribbed surfaces, V-shaped ribs are placed as the most promising among the standard-shaped ribs. This works aims to present a numerical analysis of heat transfer characteristics for forced convection inside a rectangular channel with high aspect ratio 1:10 equipped with V-shaped ribs. CFD simulations are carried out by means of Reynolds Averaged Navier-Stokes (RANS) equations, working with an air flow inside a narrow rectangular channel 120 mm wide and 840 mm long. V-shaped ribs have square cross section and they have been tested both with the tip of the V pointing downwards both upwards with respect to the channel inlet. For each configuration three different values of dimensionless pitch (10, 20 and 40) have been analysed. Comparisons between numerical and experimental data on convective global heat transfer coefficient and pressure drops are presented, showing very good agreement. Finding a properly working CFD model is useful for authors because this work, together with previous studies on 90° and 60° ribs, aims to be exploited during parametric early design-phases of new heat exchangers.

Heat transfer in the entrance region of annular laminar flow with constant and different heat flux on two walls

AbstractHeat transfer differs in the regions where the flow is developed and developing thermally. These regions can be differentiated by using the thermal entry length. Many researchers have presented correlations to determine the thermal entry length for natural and forced convection. In this study, heat transfer in the entrance region of a concentric annuli is investigated. It is accepted that beginning from the inlet of annuli the flow is developed hydrodynamically and it is developing thermally. Heat transfer is investigated where the internal or external surfaces of the annuli are at constant but different heat fluxes. The fluid velocity is assumed to be constant or radially variable. Due to thermal boundary conditions, one thermal boundary layer appears on the outer cylinder surface, another on the inner cylinder surface. The edge of two boundary layers will be adiabatic and naturally, the temperature of fluid between the two edges will be equal to free stream temperature. Transformation, Separation of Variables method, eigenvalue problem, Sturm‐Liouville system, Bessel differential equation and properties of orthogonal functions are used in solution of the problem. Exact and analytical solutions of the momentum and energy equations are presented. Velocity and temperature distributions, local Nusselt numbers and convection heat transfer coefficients are calculated for the internal and external surfaces of annuli.

AN INVESTIGATION OVER THE INFLUENCE OF RADIANT THERMAL MAT DIMENSIONS ON ITS RADIATIVE AND CONVECTIVE HEAT TRANSFER CHARACTERISTICS

For living spaces, radiant thermal mats are seen to be a good substitute for traditional heating systems. The natural convection heat transfer properties of radiant heating and cooling systems have been well studied, while the properties of radiant mats placed on surfaces have received relatively less attention. Mats of square and rectangular shapes (<i>a</i> × <i>b</i> = 0.5 m × 0.5 m, 1 m × 1 m, 1.2 m × 1.2 m, 1.4 m × 1.4 m, 1 m × 1.2 m, 1 m × 1.4 m, and 1 m × 1.6 m) are installed on the walls of an enclosure with floor dimensions <i>L</i> × <i>L</i> = 4 m × 4 m and a height of <i>H</i> = 3 m in order to address this gap in the literature. Upon analyzing the complete dataset of local convective heat transfer, it is evident that there is a steady decline in the local convective heat transfer coefficients. This decline commences at the initial point of mats, which corresponds to 3 W/m<sup>2</sup> K. This trend remains rather constant until the impact of turbulence becomes noticeably apparent, which occurs when the mat dimensions are 1 m by 1.6 m. Average convective, radiative, and overall heat transfer characteristics, which are important for building energy simulation programs, are found and correlated for different mat dimensions using the surface-to-surface (S2S) radiation model and the <i>k-ε</i> RNG turbulence model in the numerical program, with error ranges of ± 15%, ± 5%, and ± 5%, respectively.

Infrared thermography applied to the study of the local heat transfer enhancement in a wall corrugated tubular heat exchanger for food applications

Abstract The combination of global and local performance analysis represents a powerful tool for the optimal design and sizing of heat exchangers, especially when wall corrugations are employed to enhance inner convection. In this paper, both approaches are applied to a transversely corrugated pipe to propose a thermal correlation and investigate the effects of corrugation on the local heat transfer performance. For the local analysis, infrared thermographic measurements, combined with the solution of the inverse heat conduction problem in the solid wall domain, were used to evaluate the internal convective heat transfer coefficient. The combined approach offers a valuable tool for the optimal design of heat exchangers and related thermal processes, particularly in applications where knowledge or prediction of the local temperature-time history of fluid particles is critical. This is especially relevant in the food industry, where such passive heat transfer enhancement solutions are widely used.

Comparative investigation of the energy consumption and heat transfer characteristics of Uni-modal and Bi-modal slurry flow through a straight pipe

Slurry transport via pipelines is a widely used method across various industries globally. While extensive research exists on optimizing parameters for efficient slurry pipeline systems, studies on the optimal combination of two mixtures to maximize efficiency in bi-modal slurry transport remain limited. This research addresses the gap by developing a three-dimensional computational model to examine the energy, transport, and thermal characteristics of mono-dispersed and bi-modal slurries. The model compares these characteristics with the behavior of pure water flowing through a straight pipe, providing insights into how slurry composition affects flow dynamics and heat transfer. The study investigates slurry flow through a round straight pipe at Reynolds numbers ranging from 128,035 to 320,000 and volumetric concentrations (Cvf = 10–40 %). The bi-modal slurry consists of silica sand and fly ash mixtures in proportions of 85:15 and 65:35, while the uni-modal slurry has proportions of 100:0 and 0:100. Computational results indicate that the 65:35 mixture, flowing at Re = 128035 and Cvf = 10–20 % achieves the lowest pressure drop and specific energy consumption (SEC), enhancing transport efficiency. Furthermore, wall shear stress (WSS) and energy consumption increase with higher Reynolds numbers and volumetric concentrations. Notably, the 100:0 mixture achieves the highest convective heat transfer coefficient, which increases with Reynolds number and volumetric concentration.

Human body convective heat transfer coefficient under non-stationary turbulent wind

Human body convective heat transfer coefficient under non-stationary turbulent wind

Determination of Convective Heat Transfer Coefficients in the Direct Flame Heating Zone of a Continuous Hot-dip Galvanizing Unit

Determination of Convective Heat Transfer Coefficients in the Direct Flame Heating Zone of a Continuous Hot-dip Galvanizing Unit

Numerical Study of Thermal and Resistance Characteristics in the Vortex-Enhanced Tube

Heat transfer enhancement is always pursued in the industry to achieve high-performance and low-energy-consumption heat exchange devices and systems. For decades, various types of heat transfer-enhancing tubes with differing geometries and wall configurations have been developed. In this paper, the heat transfer and pressure drop characteristics of air inside an innovative heat transfer tube with regular wall dimples, namely a vortex-enhanced tube, which has a great application prospect in the gas–gas heat exchanger, are numerically studied with an experimentally validated model. The effects of the depth, axial pitch, and radial rotation angle of the dimple in the tube wall on the convective heat transfer coefficient and friction drag coefficient are comprehensively analyzed. Based on the Performance Evaluation Criteria (PEC) of the tubes, the optimal parameters of the vortex-enhanced tube are obtained. When Re ranges from 10,000 to 40,000, the comprehensive evaluation factor of the vortex-enhanced tube is 1.29 times higher than the smooth tube. Dimple pacing, dimple depth, and dimple helical angle of the optimal tube type are 8 mm, 6 mm, and 83°, respectively.

ANALYSIS OF HEAT TRANSFER TO A SHEET-TYPE HEAT EXCHANGER PLACED IN RUNNING WATER-THERMAL ENERGY EXTRACTION FROM IRRIGATION WATER

A sheet-type heat exchanger has been extensively used as a device for extracting heat from the ground soil. This heat exchanger consists of polyethylene capillary tubes bundled together in a sheet, positioned between two header pipes that serve as the inlet and outlet for circulating brine. The bundled thin tubes, with an outer diameter of 6 mm and an inner diameter of 5mm, are highly flexible, allowing for easy winding and bending. This flexibility provides significant freedom in configuring the device within the soil. This paper examines the possibility of using the same heat exchanger to collect heat from water flowing in irrigation channels and rivers. We discuss an analytical method capable of predicting the performance of the heat exchanger when it is positioned parallel to the water flow. In this analysis, we employ the concept of overall heat transfer coefficient to evaluate a single polyethylene tube. Specifically, we determine the dimensionless inner convective heat transfer coefficient, known as the inner Nusselt number, for laminar flow conditions. Meanwhile, the outer convection heat transfer coefficient is derived from forced convection correlation obtained from an isothermal flat plate. Additionally, numerical simulations are conducted to account for the influence of header pipes on the flow field and to relax the assumption of a constant outer wall temperature. We find that the analytical and numerical results generally agree. Furthermore, we compare the analytically predicted results with available experimental data, revealing agreement between the predicted and experimentally obtained heat transfer rates.

CO2 mitigation, energy matrices, and the performance analysis of N-evacuated tube collector (ETC)-integrated modified solar distiller for water/electricity co-generation using CQD nanoparticles

With advances in science and technology, nanofluids are the ultrafast heat transfer fluids which effectively empowers the distillation process. In this research article, N-evacuated tube collector (N-ETC)-coupled single slope solar still-equipped built-in-passive condenser and roof-top semitransparent photovoltaic (PV) module have been analyzed incorporating carbon quantum dot nanoparticles (CQD-NPs). The effect of volume concentration of nanoparticles (NPs) and packing factor ( βc) is tested to study the system performance (thermal and electrical). The use of CQD-NPs showed impressive enhancement in thermal conductivity (56.06%), Reynold's number ([Formula: see text]), evaporative (37.9%) and convective (76.94%) heat transfer coefficient even at moderate value of βc (0.67). Thermal energy efficiency and overall daily productivity is improved by 37.67% and 44.15%, respectively, using CQD-NPs. Moreover, thermal energy/exergy-based energy metrics and life cycle assessment has been performed for different lifespans ( n = 1 to 50 years) of the proposed system. Significant improvement in life cycle conversion efficiency [31.90% ( βc = 0), 28.91% ( βc = 0.67), 24.56% ( βc = 0.75), 32.02% ( βc = 0.85), 53.38% ( βc = 0.95)] is observed; also, enviro-economic analysis showed effective rise in annual CO2 mitigation (tons) [41.39 ( βc = 0), 104.85 ( βc = 0.67), 96.23 ( βc = 0.75), 85.14 ( βc = 0.85), 65.51 ( βc = 0.95)] and carbon credits earned using CQD-NPs. This renewable stand-alone two-in-one output system effectively meet the drinking water and electrical energy requirements of societies, and families living in arid/semiarid regions.

Numerical modelling and simulation studies of laser beam heating parameters on depth of cut and surface temperature in laser-assisted machining of Ti6Al4V

Laser-assisted machining (LAM) is a hybrid machining process which uses a laser heat source to heat the narrow zone of a work part material ahead of the cutting tool. Surface temperature and heat-affected depth (effective depth of cut) achieved during localised laser heating play an important role in the machinability of the workpiece. In the current work, moving laser heat source simulation studies are performed using COMSOL Multiphysics software. The effect of laser power, beam diameter, scan speed, convective heat transfer coefficient and absorptivity are studied by simulating different test cases. Results show that the average maximum surface temperature increases when the laser power is increased from 100 W to 160 W by 55%. It increases with the increase in absorptivity from 0.3 to 0.39 by 18%. The average maximum surface temperature decreases when the laser diameter is increased from 1 to 2.5 mm by 46%. It decreases with the increase in scan speed from 50 to 200 mm/min by 5%. In the case of effective depth of cut, similar trends were observed with the variation of the above parameters. Further, the variation of convective heat transfer coefficient from 5 to 20 W/m-K does not affect the average maximum surface temperature and effective depth of cut. It was found that the laser power has a significant effect on average maximum surface temperature and effective depth of cut compared to other parameters.

STUDI EKSPERIMEN PENINGKATAN PERPINDAHAN KALOR FLUIDA NANO CuO /AIR PADA VERTICAL HELICAL MICROFIN TUBE

The way to increase heat transfer is to use nanofluids and expand the heat transfer area. This research studied the thermal performance, convection heat change coefficient, and pressure drop in a double pipe heat exchanger experimentally in a vertically arranged helical microfin tube heat exchanger. Hot water flows on the side of the annulus. In contrast, nanofluid CuO/distilled water concentration of 0.05 vol% flows laminarly in the inner pipe of the microfin tube with cross-flow and parallel-flow arrangements. The result is convection heat transfer coefficient enhancement and thermal performance factor compared to the base fluid. Counter flow improves heat transfer better than parallel flow. This research contributes to the use of helical microfin and nanofluids to increase heat transfer in heat exchangers used in industrial processes.

Convective heat transfer improvement for a horizontal grooved surface by the piezo-fan integration

In this study, the thermal and hydraulic performances of horizontal grooved surfaces subjected to the piezo-fan ( PF) is numerically investigated. A total of four grooved surfaces with varying dimensionless edge length of the square groove cross-section ( EL = 0.25, 0.5, 0.75 and 1) and a corresponding baseline flat surface ( EL = 0) are considered. Two kinds of numerical models are utilized to capture the turbulent characteristics and temperature distribution. The dynamic mesh technique and user-defined function are employed to descript the vibration trajectory of the PF. The focus of this study is on assessing the impact of PF in the design of an active heat sink. It is confirmed that, due to the change in surface geometry, the flow cannot follow the surface contour perfectly, leading to a pressure drop and a separation of flow from the grooved surface. The integration of PF into a horizontal grooved surface is a highly efficient means for improving heat transfer performance. Its role in heat transfer enhancement is most significant when the dimensionless edge length is EL = 0.75. With the presence of PF, the effective area-averaged convective heat transfer coefficient ( harea,eff) both on flat and grooved surfaces are generally larger than the corresponding one without the PF. Of particular is that the harea,eff of both types of target surfaces reach their maximum within the specified zone S4 (4 APP × 4 WPF), with the grooved surface being approximately 14% to 28% higher than the flat surface. Moreover, the numerical simulation method has been validated through the comparison with a simple heat transfer experiment.