Value Of Convective Heat Transfer Coefficient Research Articles

New generalized expressions for forced convective heat transfer coefficients across the flat plate

This study presents a novel generalized model for forced convective heat transfer coefficients (CHTC) over flat plates, addressing key limitations in existing models. Current expressions often provide averaged values and fail to capture the local variations in heat transfer, leading to inaccuracies, particularly in transitional zones. Moreover, there is no consensus on boundary layer classifications, which further complicates the development of reliable CHTC models. To address these issues, we conduct extensive CFD simulations to systematically analyze the combined effects of plate length (L), wind velocity (v), position along the plate (x), and temperature difference (Δt) on local CHTC. The simulations span a wide range of conditions (0.25 m ≤ L ≤ 8 m, 3 m/s ≤ v ≤ 25 m/s, and 5 °C ≤ Δt ≤ 80 °C), creating a comprehensive database of local CHTC values. Based on this data, a new generalized CHTC expression is derived as a function of L, v, x, and Δt, accurately predicting CHTC across laminar, transitional, and turbulent zones of flat plats. The accuracy of proposed CHTC expression is verified through thorough in-sample and out-of-sample evaluations, achieving R2 values between 0.973 and 0.999, with maximum deviations ranging from 2.6 % to 8.5 %. These research methods and conclusions can provide valuable references for optimizing thermal processes in applications such as industrial heat exchangers, electronics cooling, and building heating systems, where precise heat transfer modeling is critical for enhancing efficiency and system design.

Developing a Chained Simulation Method for Quantifying Cooling Energy in Buildings Affected by the Microclimate of Avenue Trees

This paper introduces a methodology aimed at bridging the gap between building energy simulation and urban climate modeling. A coupling method was developed through the Building Control Virtual Test Bed (BCVTB) and applied to a case study in Taipei City, Taiwan, to address the microclimate factors of street trees crucial to cooling energy consumption. The use of the Urban Weather Generator for weather file modification revealed a 0.63 °C average air temperature disparity. The coupling method emphasized the importance of accurate wind speed and convective heat transfer coefficients (CHTCs) on building surfaces in determining cooling energy. The results indicated that elevated CHTC values amplify heat exchange, with higher wind velocities playing a crucial role in heat dissipation. The presence of street trees was found to significantly reduce heat flux penetration, leading to a reduction in building surface temperatures by as much as 9.5% during hot months. The cooling energy was lowered by 16.7% in the BCVTB simulations that included trees compared to those without trees. The EnergyPlus-only simulations underestimated the cooling energy needs by approximately 9.3% during summer months. This research offers valuable insights into the complex interactions between buildings and their environments. The results highlight the importance of trees and shading in mitigating the heat island effect and improving energy-efficient urban planning.

An Investigation into The Enhancement of Heat Transfer in Roughened Ducts of Solar Air Heaters

One of the most crucial tools for the process of transforming solar energy into thermal energy is a solar air heater. Thanks to its low cost and ease of installation, solar air heaters have quickly become one of the most popular and widely used methods of harvesting solar energy. Low convective heat transfer coefficient values between the absorber plate and the air significantly reduce the solar air heater's thermal efficiency. This is because absorber plates are used in solar air heaters. As a consequence, the absorber plate heats up, releasing a great deal of thermal energy into the surrounding space. This article presents the findings of a study that used computational fluid dynamics to investigate how heat is transferred in a solar air heater. The work for this project was done by the author (CFD). Researchers are now investigating the impact of the Re on the Nu. Commercial software known as ANSYS FLUENT 20 may be used to analyse and visualise the flow that happens across the duct of a solar air heater. This programme falls under the category of finite volume software. Using the programme helps get the job done.

Experimental Study on Indirect Liquid Cooling Performance of Metal 3D-Printed Cold Plates for Battery Thermal Management

Two new cold plates manufactured via metal 3D printing were experimentally investigated for thermal performance analysis in indirect liquid cooling operations; then they were compared to the traditional cold plates. Experiments were performed with different coolant inlet temperatures (15.7 °C and 24.5 °C) and ambient air velocities (0.5 m/s and 3 m/s) at tropical conditions; hereby, the impact of high dew point temperatures at tropics was also investigated. Body-centered cubic (BCC) and pillar elements were applied in the cooling cavity of the cold plates. The results showed that the target surface temperature in both BCC- and pillar-filled plate designs was maintained below the limits at the lower inlet temperature. However, at the higher inlet temperature, the temperature was only maintained below the limit when the ambient air velocity was 3 m/s. The convective heat transfer coefficient at the inlet temperature of 15.7 °C was found 1.5 and 2.5 times higher than the convective heat transfer coefficient value at the inlet temperature of 24.5 °C for the pillar- and BCC-filled plates, respectively. The performance evaluation criterion values were found in the range of 1.2 − 2.4, which depended on the operating conditions and were already higher than the referenced studies in the literature.

Effect of flow rate and CNM concentration in nanofluid on the performance of convective heat transfer coefficient

The aim of this study is to investigate the thermal performance of water-based Carbon Nano Material (CNM) nanofluids, particularly in the convective heat transfer coefficient (h) parameter, and to analyze the effects of nanoparticle concentration and flow rate on an experimental heat transfer system. To achieve this, the researchers prepared the nanofluid through a 2-step method. In the first step, we mixed the nanoparticles with the base fluid using a magnetic stirrer to ensure homogenization, with the CNM-Water ratio set at 30, 75, 120, and 150 ppm. In the second step, we characterized the prepared samples, determining their size, composition, and particle morphology through PSA and SEM-EDX examination, as well as measuring their density (ρ) and specific heat (Cp). The experiments were carried out using a flow rate simulation test rig with a data acquisition system, at different flow rates of 0.2, 0.6, 1, and 1.4 L/min. Temperature (T) was measured at the inlet, outlet, and heater to determine the convective heat transfer coefficient value. The study successfully investigated the direct impact of CNM concentration and flow rate, with the results showing a consistent and expected value of the coefficient compared to previous studies. The highest coefficient value was observed at 150 ppm CNM-Water ratio and a flow rate of 1.4 L/min, with a value of 1825.37 W/(m2.K).

The effects of topological configuration and geometric parameters on heat transfer and fluid flow characteristics of lattice-based heat sinks

Traditional pin-fin heat sinks could provide only thermal exchange. Whereas the topology and geometry of lattice-based heat sinks make them suitable for simultaneous thermal and structural applications. In this work, we numerically demonstrate the convective heat dissipation performance of periodic pyramidal and tetrahedral lattice-based heat sinks. The motivation for the study is to discover the effects of topological and geometrical parameters of the lattice structure on the heat and fluid flow characteristics of lattice-based heat sinks which are not yet presented in the literature. Hence, we consider pyramidal and tetrahedral lattice topologies with two different L/D ratios (8.8 and 5). Three-dimensional numerical simulations based on the finite volume method were carried out at constant pumping power with water as a coolant for applied heat flux in the range of 10–50 W/cm2. The thermo-hydraulic performance is characterized by the efficiency index and area goodness ratio as a function of pumping power in the laminar regime. The convective heat transfer coefficient values range from 2129 to 6510 W/m2 K for the operating conditions considered in this study. Also, in terms of overall combined thermo-hydraulic performance, the tetrahedral heat sink with the L/D ratio of 5 was found to be 50–53%, 18–25%, and 22–30% more efficient than the P8.8, T8.8, and P5 heat sinks, respectively, for the range of pumping powers considered. The thermo-hydraulic characteristics presented in this study enable the design of lattice-based heat sinks for specific applications with collective pressure drop and heat transfer constraints.

Experimental investigation of convective heat transfer in the aircraft cabin environment at low air pressure

The heat transfer capacity of the cabin environment under low air pressure is significantly different from that under normal pressure. The existing research on the low-pressure environment of aircraft cabins has not provided a reliable convective heat transfer correlation to guide the design of the cabin thermal environment. This investigation evaluated the heat transfer in an actual aircraft cabin under different pressures. A laboratory test was designed in a cabin mockup with controllable boundary conditions for air temperature and pressure. Based on the measurement results, the quantitative effects of low pressure on the convective heat transfer coefficient (CHTC) of the heating surface were analyzed. According to the experimental results, the CHTC of the horizontal surface in natural convection increased from 3.26 W/(m2·K) to 5.10 W/(m2·K) when the heat flux increased from 100 W/m2 to 400 W/m2, and the CHTC dropped by 23–30% when the pressure dropped from 101 kPa to 61 kPa. With the same heat flux, the average CHTC value for the vertical plate was slightly lower than that for the horizontal heating plate under normal pressure, and the trend became significant as the air pressure decreased. In general, the average CHTC for forced convection was substantially higher than that for natural convection. Consequently, correlation equations for calculating natural and forced convection on horizontal and vertical plates under low air pressure were proposed, which will aid the design of the cabin thermal environment.

Combined Effect of Brick Surface Roughness and Lattice Setting Density on Brick Firing in Tunnel Kilns

This paper investigates the combined effect of the kiln or brick surface roughness and the brick lattice setting density on the fluid flow and heat transfer characteristics in tunnel kilns. The flow uniformity, pressure drop, convective heat transfer coefficient (CHTC), and pumping power are studied. A high-density setting (HDS), which comprises 768 bricks, and a low-density setting (LDS), which comprises 512 bricks, are tested for kiln boundaries and brick surface roughness levels of 0, 1, 2, 3, and 4 mm. The investigation is conducted using a 3D-CFD model with the k-ω turbulence model. The surface roughness changes from 0 to 4 mm for either kiln walls or bricks while fixing the other. The results show that increasing the tunnel kiln surface roughness from 0 to 4 mm increases the pressure drop of both the HDS and LDS by about 13.5%. It also increases the established CHTC value of the LDS more than the HDS by about 23% for all tested roughness levels. Changing the brick surface roughness from 0 to 4 mm increases the pressure drop and CHTC value for the LDS more than for the HDS by about 10% and 12%, respectively. Additionally, the total heat transfer rate-to-pumping power ratio for the LDS is larger than for the HDS by 17.4% for smooth bricks and 23.1% for the brick roughness of 2 mm, i.e., the brick roughness provides a greater advantage to the LDS. The results confirm that the LDS for rough and smooth bricks loaded in tunnel kilns attains a better brick quality, a higher heat transfer rate, and a lower pumping power than the HDS.

Drying quality and energy consumption efficient improvements in hot air drying of papaya slices by step-down relative humidity based on heat and mass transfer characteristics and 3D simulation

Convective hot air drying technology is widely used for fruits and vegetables drying process due to its simplicity in usage, small investment, and convenience in operation. However, it is generally considered energy-intensive process with lower drying efficiency and quality because of unprecise control of drying process. In order to solve these problems, step-down relative humidity (RH) hot air drying technology was applied for papaya drying to improve drying efficiency and quality and reduce energy consumption. The effect of constant RH (20%, 30%, 40%, and 50%) and step-down RH of 50% with different holding times (10, 30, 60, and 90 min) and then decrease to 20% on heat and mass transfer process, drying quality, and energy consumption was examined. Results showed that decreasing RH contributed to improvement in the drying rate (DR). Compared to constant RH of 20%, the drying time under initial RH of 50% for 30 min followed by 20% RH shortened by 21.4%. Additionally, under this RH control strategy, the rehydration ratio (RR) and energy consumption achieved their maximum (7.71 ± 0.06) and minimum value (0.015 ± 0.001 kJ/g), respectively. During initial drying stage, high RH improved convective heat transfer coefficient (ht ) values and material temperature increased quickly. Furthermore, high RH contributed to maintaining pore structure formation, which benefited moisture diffusion. When RH was reduced, low RH enhanced convective mass transfer coefficient (hm ) significantly, resulting in rapid diffusion of internal moisture at high temperature, as well as rapid moisture evaporation from the surface. Therefore, step-down RH hot air drying was verified to improve drying of papaya slices, making it energy efficient process. The findings in current work indicate step-down relative humidity strategy is a promising technique to enhance hot air drying process, quality, and energy efficiency of papaya slices.

Preliminary analysis of the influence of environmental boundary conditions on convective heat transfer coefficients

In this contribution, a hot box apparatus was used to investigate convective heat transfers through an experimental setup composed of a heat-flow sensor, air and surface temperature probes installed on the wall tested in the hot box and a hot-wire anemometer. The air near the wall was indirectly moved by means of a forced convection system through which the air velocity can be modified to simulate different environmental conditions. The hot box apparatus was used in order to investigate the influence of the air velocity on internal convective heat transfer coefficients (hc), under natural/mixed convection conditions. The Standard ISO 6946 suggests a typical internal convective heat transfer coefficient equal to 2.5 W/m2K when horizontal heat fluxes occur. No detailed information about this value are reported in the Standard. So, the final aim of this study is to assess the applicability of the internal convective heat transfer coefficient value proposed by ISO 6946 under different air velocity values, quantifying how hc varies as air velocity changes.

Homogeneous and Multiphase Analysis of Nanofluids Containing Nonspherical MWCNT and GNP Nanoparticles Considering the Influence of Interfacial Layering.

The practical implication of nanofluids is essentially dependent on their accurate modelling, particularly in comparison with the high cost of experimental investigations, yet the accuracy of different computational approaches to simulate nanofluids remains controversial to this day. Therefore, the present study is aimed at analysing the homogenous, multiphase Eulerian–Eulerian (volume of fluid, mixture, Eulerian) and Lagrangian–Eulerian approximation of nanofluids containing nonspherical nanoparticles. The heat transfer and pressure drop characteristics of the multiwalled carbon nanotubes (MWCNT)-based and multiwalled carbon nanotubes/graphene nanoplatelets (MWCNT/GNP)-based nanofluids are computed by incorporating the influence of several physical mechanisms, including interfacial nanolayering. The accuracy of tested computational approaches is evaluated by considering particle concentration and Reynolds number ranges of 0.075–0.25 wt% and 200–470, respectively. The results demonstrate that for all nanofluid combinations and operational conditions, the Lagrangian–Eulerian approximation provides the most accurate convective heat transfer coefficient values with a maximum deviation of 5.34% for 0.25 wt% of MWCNT–water nanofluid at the largest Reynolds number, while single-phase and Eulerian–Eulerian multiphase models accurately estimate the thermal fields of the diluted nanofluids at low Reynolds numbers, but overestimate the results for denser nanofluids at high Reynolds numbers.

CFD-based calculation method of convective heat transfer coefficient of spiral bevel gear in intermediate gearbox under splash lubrication

Purpose This paper aims to propose a numerical method to calculate the convective heat transfer coefficient of spiral bevel gears under the condition of splash lubrication and to reveal the lubrication and temperature characteristics between the gears and the oil-air two-phase flow. Design/methodology/approach Based on computational fluid dynamics, the multiple reference frames (MRF) method was used to simulate the rotational characteristics of gears and the motions of their surrounding fluid. The lubrication and temperature characteristics of gears were studied by combining the MRF method with the volume of the fluid multiphase flow model. Findings The convective heat transfer coefficient can be improved by increasing the rotational speed and the oil immersion depth. Moreover, the temperature of the tooth surface having a large convective heat transfer coefficient is also found to be low. A large convection heat transfer coefficient could lead to a good cooling effect. Originality/value This method can be used to obtain the convective heat transfer coefficient values at different meshing positions, different radii and different tooth surface positions. It also can provide research methods for improving the cooling effect of gears under the condition of splash lubrication. Peer review The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-07-2020-0233/

A finite element simulation model of convective heat-assisted single-point incremental forming of thermoplastics

The objective of this research is to simulate the effects of temperature on the stress, strain, and forming forces in the single-point incremental forming process of polymers through the development of a FEA model. Currently available SPIF simulation models are limited to room temperature and are not suited to processes that use localized heating to increase formability. The model developed in this research consists of three key components: (a) simulated temperature spot using a distribution of convective heat transfer coefficients (HTC) values, (b) temperature-dependent material behavior using the Three Network model (TNM), and (c) and the integration of thermal and mechanical behavior of the SPIF process. The forming of a polycarbonate cone with a constant wall angle of 45 degrees is simulated using the model under two conditions: (i) room temperature SPIF and (ii) CHASPIF using an external hot air source of 250 °F (121.11 °C). The accuracy of the predicted deformation and temperatures varies in the different zones. Inside the formed wall, the temperature distribution is predicted within ± 5 °C for CHASPIF, and deformation within 0.766 mm for CHASPIF and 0.786 mm for SPIF compared to experimental results. Future research will be focused on improving the accuracy of the model and evaluating process limits for more complex shapes. The simulation model is ultimately intended to be used for model-based manufacturing.

Influence of the convective coefficient on the determination of thermal transmittance through outdoor infrared thermography

Several studies and correlations for the convective heat transfer coefficient (CHTC) are available in literature and in handbooks, depending on type of convection, wind speed range or on the test rigs from which they are derived.The importance of accurate evaluation of CHTC, especially at the building façades, has been highlighted in the recent years, due to the need for reliable measurement of buildings’ heat transfer capability. This study aims at underlining the importance of proper CHTC values for the determination of the building envelope thermal transmittance (U-value) via infrared thermography (IRT). To this scope, firstly an overview on convective heat transfer coefficient is given; then, some CHTC models, chosen from literature, are analyzed at different wind speed classes. Subsequently, such models are employed in two formulas proposed in literature for the U-value measurement via IRT, by using data from previous experimental campaign carried out in controlled environment. Results were compared, and significant deviations were found: one of the employed approaches and formulation is less sensitive to the correlation adopted for the convection expression, amongst those considered and in the wind speed range analyzed. This constitutes an advantage, since one of the weak points of the IRT method is the convection expression itself.

Determination of heat transfer coefficients and drying kinetics of red chilli dried in a forced convection mixed mode solar dryer

Solar drying is an efficient and economical way of drying of agricultural products as compared to any conventional dryers, and also in solar dryers, the products are dried under the hygienic condition in an enclosed chamber. The heat transfer coefficients play a significant role in drying operation. The temperature difference observed between the drying air and agriculture products varies with the heat transfer coefficient. Anexperimental investigation on the drying of red chilli was performed in a forced convection mixed-mode solar (FCMS) dryer integrated with an energy storage system. This paper focusses on studying the effect of moisture evaporation rate on the heat transfer coefficient of red chilli dried in the FCMS dryer. A correlation was obtained in the form of Nu = C(Rem Prn) from the experimental data with C = 1.146 and m = 0.296. The experimental drying data obtained are used to evaluate the convective heat transfer coefficient (CHTC) hc which are further used in the calculation of evaporative heat transfer coefficient (EHTC) he. The average values of CHTC and EHTC obtained were 1.63 and 49.7 W*m−2*C−1, respectively. The paper also focusses on selecting a suitable drying kinetic model for investigating the drying behaviour of red chilli in the FCMS dryer, which can be used in the design and optimization of the dryer.

The investigation with exergetic indicators and heat transfer parameters of solar drying process of white mulberry

ABSTRACT In this paper, the effects of useful energy ratio, some exergetic indicators on the performance of a thin-layer solar drying system by using the experimental data in the literature for white mulberry, were investigated. In addition, the forced convection heat transfer parameters of white mulberry were evaluated. The results demonstrated that the energy utilization ratio, exergetic efficiency and waste exergy ratio varied between 7.142–47.142% and 18.33–98.62%, and 1.37–81.66%, respectively. The exergetic sustainability indexes and environmental impact factors changed from 0.224 to 8.079 and 0.0139 to 4.453, respectively. The values of improvement potential ranged from 0 to 0.053 kW. The convective heat transfer coefficient values for white mulberry were between 18.947 and 19.040 W/m2°C.

Thermal Analysis of a Microhotplate - An Experimental Study

In the present work, experimental and numerical thermal analysis of a microhotplate were performed to evaluate the heat transfer mechanism at small Rayleigh number ( $Ra$ ). The convective and radiative heat transfer coefficient values were evaluated for microhotplate mounted over two different mounting positions (namely model-1 & model-2). At different heat generation ( $Q$ ) values, it was found that the average convective coefficient value was less than $10\frac {W}{m^{2}K}$ , irrespective of the microhotplate mounting position. Comparison of both models at a similar $Q$ ( $\simeq 0.69W$ ) value, reveals that the radiative heat transfer contribution was 33% & 68% for model-1 and model-2 respectively. Based on the results obtained in the present study, a correlation was developed using regression analysis for the convective Nusselt number in terms of Rayleigh number ( $8.84\times 10^{-4}\le \:Ra\:\le 4.94\times 10^{-3}$ ), and the convective Nusselt number obtained from the correlation and present study are within ±17%.

Experimental investigation and finite element modeling of localized heating in convective heat-assisted single-point incremental forming

A method for simulating localized convective heating in convective heat-assisted single-point incremental forming (CHASPIF) is developed in this research. Localized convective heating in CHASPIF is a type of generic spot heating problem, which is found in manufacturing processes such as welding and laser heating. Existing approaches are not suitable for convective heating, as the heat flux is not constant and depends on the temperature difference between the source and the target surface. In this study, the heat load is simulated by applying a distribution of convective heat transfer coefficient (HTC) values that are displaced and oriented along the target surface to simulate localized heating. The HTC values are experimentally determined using a heat flux sensor and approximated as a series of sectors to create a heat spot profile which is implemented in ANSYS. The heat load profile is demonstrated using two scenarios in which the tool moves in a spiral and a linear path, and is able to predict temperatures with a maximum error of ± 4.358 °C. This method can be used as part of a temperature-dependent simulation model for heat-assisted single-point incremental forming and can also be used in simulating generic spot heating applications. In the future, the accuracy of the model will be improved by considering the variation of HTC values with deformation of the blank, non-linear transient response of the hot air source, reducing discretization errors, and uncertainties in the HTC data collection.

Experimental investigation of convective heat transfer coefficient on nano particles mixture used in automobile radiator based on mass flow rate

Experimental investigation carried about improvement of convective heat transfer coefficient of radiator by using 75% of Al2O3 and 25% of CeO2 combination of nano particles for automobiles. There are different conditions of mass flow rate (0.02 kg/s, 0.04 kg/s, 0.06 kg/s, 0.08 kg/s and 0.1 kg/s) and various volumes of nano particles combination (0.2%, 0.4%, 0.6%, 0.8% and 1%) considered for this investigation. Percentage of nano particle volume and mass flow rate also created the considerable impact in the heat transfer rate and convective heat transfer coefficient values at constant inlet temperature.

CFD simulation of external CHTC on a high-rise building with and without façade appurtenances

Many high-rise buildings have intricate façade appurtenances such as balconies, mullions, or egg-crates shading elements. These façade systems interact with the external microclimate in a complex manner that affects the convective heat transfer coefficient (CHTC) significantly. In this study, CFD and heat transfer simulations are carried out for buildings with balconies, vertical shading element, egg-crates, and a smooth façade. Comparisons of local-CHTC distribution between buildings with and without the façade appurtenances for rooms on different floor heights and located at a corner or a center region of the building are performed. Two equation SST κ-ω turbulence modeling is used for momentum closure. The result of this study shows that the local and surface-averaged CHTC values at the surfaces of each building are dependent on building aerodynamics and forms of the façade appurtenances. For instance, for a 100 m tall building case study with rectangular floor plan, having an egg-crate shading with a depth of 1 m decreased the local-CHTC of a room around the corner-zone by 35%, 37%, and 38% on the 1st, 15th, and 30th floors, respectively compared to a room in building a smooth façade. Thus, necessitating detail local-CHTC variations and wind directionality effect assessments especially on glazed buildings characterized by weak thermal performances.