Convective Heat Transfer Coefficient Research Articles

Experimental evaluation of the wind convection heat transfer on the glass cover of the single‐slope solar still

AbstractAccurate evaluation of the wind convection heat transfer coefficient (hw) for solar‐based systems is essential, especially for solar desalination systems. Thermal behavior and productivity of solar stills are highly affected by the external heat loss through the glass cover. This paper describes a new experimental approach to estimate the hw on the glass cover of the conventional single‐slope solar distiller (CSS). Indoor experiments have been conducted under steady‐state conditions for a wind speed between 0 and 3 m/s. The hw has been evaluated through an energy balance performed on the distiller's glass cover. The results showed that increasing the wind speed increases the hw (from 5.64 to 31.57 W/m2 K) and enhances the distillation rate (from 5.28 to 7.61 mL/min). A new relationship for the hw was proposed for the CSS and compared with the experimental data available in the literature. The comparison shows that the obtained results are close to the data from solar systems, with a deviation ranging from 27.4% to 37%. However, a significant deviation was obtained with earlier models derived from flat plates (from 29.5% to 59%).

Accuracy in evaluating convective heat transfer coefficient by RANS CFD simulations in a rectangular channel with high aspect ratio and 60° tilted staggered ribs

A numerical analysis of heat transfer characteristics of an air flow through a narrow rectangular channel of 1:10 aspect ratio with ribbed surfaces has been carried out, by means of a CFD commercial code, exploiting Reynolds Averaged Navier-Stokes (RANS) equations. The channel is 120 mm wide, 840 mm long, with 60° tilted staggered ribs. Ribs have a square cross section with two different side heights (2 mm and 4 mm) and three different values of dimensionless pitch (10, 20 and 40). The numerical results have been compared with experimental data obtained by the authors inside a ribbed channel both with same geometry and operating conditions as one numerically modelled. Agreement between numerical and experimental data on convective global heat transfer coefficient, i.e., averaged over the whole channel, is discussed for the six different configurations considered. Moreover, performances are presented by considering at the same time both heat transfer enhancement and pressure-drop penalization, highlighting strengths and weaknesses per each configuration. This work is aimed at finding a suitable configuration of a CFD model with RANS that will allow authors to apply it to the range of dimensionless pitches and side heights during early design-phases of parametric studies.

EXPERIMENTAL ANALYSIS OF HEAT TRANSFER AND THERMAL PERFORMANCE OF PARABOLIC TYPE SOLAR COLLECTOR WITH RIBBED SURFACE TEXTURE FOR CLEAN ENERGY EXTRACTION

This paper examines the performance of a parabolic type solar collector (PTSC) that uses both plain tube and ribbed surface textured tube channels for elevating the water temperature used for various applications. The performance of a solar water heater is evaluated experimentally. During the experiment, the solar radiation intensity and the feed water flow rate of 1.0 kg/min to 5.0 kg/min in steps of 1 kg/min are taken into consideration for analyzing the effect of ribbed textured tubes on the thermal effectiveness, frictional factor, convective transfer of heat, Reynolds number, and the Nusselt number of the PTSC. Furthermore, the overall performance of the PTSC is analyzed considering the above thermo-physical parameters. Based on the result of this study, at a flow rate of 3.0 kg/min, the thermal efficiency is found to be enhanced by about 28.25%, the friction factor is augmented by about 0.23%, the convective heat transfer coefficient is improved by 24.22%, and the Nusselt number is increased by about 26.32%. On average, an overall improvement in the performance of 8.25% is observed for the ribbed textured tube than that of the plain tube. The experimental error analysis shows that the standard deviation for both plain and ribbed textured tubes is in the range of 3.2, which is in the acceptable limit.

Experimental Measurement and Modeling Analysis of the Heat Transfer in Graphene Oxide/Turbine Oil Non‐Newtonian Nanofluids

Heat transfer characteristics of graphene oxide (GO)/turbine oil as a non‐Newtonian nanofluid are assessed both experimentally and numerically in this paper. To do so, 0.2, 0.3, 0.5, and 1 mass percent (wt%) of GO is homogeneously dispersed in the base liquid. First, the specific heat capacity, thermal conductivity, viscosity, and density of the synthesized nanofluids are measured using standard laboratory methods. After that, constants of the shear stress equation are determined through the nonlinear regression of the rheology data on the power law model. Finally, the heat transfer from turbine blades with a constant surface temperature to the coolant nanofluid is investigated using mathematical modeling. The results suggest that while the nanofluid density, viscosity, and thermal conductivity increase by increasing the nanoparticle concentration by 0.57%, 7.07%, and 18.89% in succession, respectively, and its specific heat capacity decreases by 0.54%. Moreover, both the convective heat transfer coefficient and the temperature profile in the considered nanofluids depend on the average velocity and Reynolds number. Furthermore, the convective heat transfer coefficient increases by 5.5%, 9.5%, 14%, and 17% in exchange for 0.2, 0.3, 0.5, and 1 wt% of GO nanoparticles in the base liquid, respectively.

Experimental investigation on cooling tower performance with Al<sub>2</sub>O<sub>3</sub>, ZnO and Ti<sub>2</sub>O<sub>3</sub> based nanofluids

<p>This study deals with an experimental investigation of the thermal performance of a prototype mechanical wet cooling tower with a counter flow arrangement. Different volume concentrations ranging from 0.18 to 0.50 vol.% of stable Al oxide (Al<sub>2</sub>O<sub>3</sub>), Zn oxide (ZnO), and Ti oxide (Ti<sub>2</sub>O<sub>3</sub>) nanoparticles of 80, 35, and 70 nm diameter were considered. Water was taken as a base fluid, and the experiment was carried out at 60, 70, and 80 ℃, respectively, in laboratory conditions. The study revealed that an increase in the volume concentration of the nanofluids increased the cooling range, cooling efficiency, convective heat transfer coefficient, tower characteristic called number of transfer unit (NTU), and effectiveness of the cooling tower compared with water at the same mass flow rate and inlet temperature. However, increasing the volume concentration increased the viscosity of the nanofluids, leading to an increase in friction factor. For instance, for 0.18% volume concentration of ZnO, at an inlet water temperature of 66.4 ℃ and water/air (L/G) flow ratio of 1.93, the cooling range increased by 3.62%, cooling efficiency increased by 33.3%, and NTU increased by 50.5% compared with fresh water (FW).</p>

Temperature digital twins model for blueberry pre-cooling based on micro-cluster method

Abstract Objectives In order to improve the prediction accuracy of forced-air pre-cooling for blueberries, a mathematical model of forced-air pre-cooling for blueberries based on the micro-cluster method was established. Materials and Methods In order to determine the optimal micro-cluster model parameters suitable for forced air pre-cooling of blueberries, three factors controlling the micro-cluster geometry parameters were evaluated by 7/8 pre-cooling time, uniformity, and convective heat transfer coefficient. Results It was found that the optimal values of the number of micro-clusters (n3), the distance between individual units within a micro-cluster (a) and the distance between micro-clusters (c) were 3, 0.75, and 0.2, respectively. Under these optimal values, the temperature error of the micro-cluster method remained below 1 °C, achieving highly accurate temperature predictions during the blueberry pre-cooling process. The results showed that the micro-cluster method effectively solved the challenges of complex configuration, long simulation time, and low accuracy compared to the porous medium and equivalent sphere methods. Conclusion Based on the above analysis, it can be concluded that the micro-cluster method provids a theoretical basis for optimizing forced-air pre-cooling processes and making informed control decisions.

Numerical analysis of the binder angle effect on convective heat transfer and pressure drop in drilled-hollow sphere architected foams

Due to their capability of generating customized microstructures, additive manufactured cellular materials are promising to being employed in heat transfer devices. To this aim, among printable cellular materials Drilled-Hollow Spheres Architected (DHSA) foams are investigated. However, at the present status, limited data on pressure drop and heat transfer in DHSAs are available. Starting from hollow spheres, a metal DHSA foam is generated with CAD software in this study. Forced air convective heat transfer in the foam is investigated numerically, under the assumptions of air incompressible laminar flow and uniform wall heat flux from the solid to the fluid phase. Mass, momentum and energy equations in the fluid region are written and solved numerically, for various values of the foam binder angle and the velocity of the inlet air. The convective heat transfer coefficients, the pressure drop and the friction factor are predicted. The effects of the binder angle and the air inlet velocity on heat transfer and pressure drop are highlighted.

Analyzing The Influence of Diameter and Winding on Heat Transfer Efficiency in Spiral Tube Heat Exchangers: A CAD-Integrated CFD Study Using Solidworks Flow Simulation

Spiral tube heat exchangers (STHE) are coiled metal devices with two fluid channels around a central core, enabling counterflow or parallel flow of gases, liquids, or both. Compared to traditional straight-tube heat exchangers, STHEs offer a larger heat transfer surface area. This study used Computational Fluid Dynamics (CFD) simulation integrated with Computer Aided Design (CAD) to investigate STHE’s heat transfer performance. The STHE dimensions, a 12-mm copper tube, and a 10-inch PVC shell were adopted from a previous study. Cold and hot water at 20°C and 70°C flowed in parallel at specific flow rates. The objective was to explore the impact of STHE dimensions on heat transfer efficiency and performance. The parameters varied were the internal diameter of the copper tube and the number of spiral coil windings. Results revealed that changing the spiral heat exchanger’s diameter affected the heat transfer rate and coefficient. Larger diameters reduced efficiency due to lower flow velocities and convective heat transfer coefficients. The number of windings significantly affected heat transfer performance, with winding 5 demonstrating the highest rate and winding 7 showing the highest coefficient. CFD analysis reliability was validated by convergence with analytical solutions for heat transfer simulations with varying diameters and windings.

Barriers and variable spacing enhance convective cooling and increase power output in solar PV plants

When the temperature of solar photovoltaic (PV) modules rises, efficiency drops and module degradation accelerates. Thus, it is beneficial to reduce module operating temperatures. Previous studies of solar power plants have illustrated that incoming flow characteristics, turbulent mixing, and array geometry can strongly impact convective cooling, as measured by the convective heat transfer coefficient h. In the fields of heat transfer and plant canopy flow, previous work has shown that system-scale arrangement modifications—e.g., variable spacing, barriers, or windbreaks—can passively alter the flow, enhance turbulent mixing, and influence convection. However, researchers have not yet explored how variable spacing or barriers might enhance convective cooling in solar power plants. Here, high-resolution large-eddy simulations model the air flow and heat transfer through solar power plant arrangements modified with missing modules and barrier walls. We then perform a control volume analysis to evaluate the net heat flux and compute h, which quantifies the influence of these spatial modifications on convective cooling and, thus, module temperature and power output. Installing barrier walls yields the greatest improvements, increasing h by 3.4%, reducing module temperature by an estimated 2.5 °C, and boosting power output by an estimated 1.4% on average. These findings indicate that incorporating variable spacing or barrier-type elements into PV plant designs can reduce module temperature and, thus, improve PV performance and service life.

FLOW AND HEAT-TRANSFER CHARACTERISTICS IN SMALL-DIAMETER TUBE BUNDLES WITH A STAGGERED LAYOUT: AN EXPERIMENTAL STUDY

While past research has primarily focused on heat transfer in larger diameter tubes, the capabilities of small-diameter tubes have often been estimated using correction factors. However, with the evolution of compact heat exchangers, conducting dedicated studies on small-diameter tube bundles has become increasingly crucial to achieve more precise conclusions. An experimental research on flow and heat-transfer characteristics of staggered tube bundles with different tube diameters (2, 3, and 5 mm) is conducted. In the experiment, the number of rows (4-12), the mass rate of the air (0.06-0.18 kg/s), and the transverse tube pitch (<i>S</i><sub>1</sub>/<i>d</i> = 2, <i>S</i><sub>1</sub>/<i>d</i> = 3) are variables to study the characteristics of the airside flow resistance and heat transfer. The three main conclusions of the experimental results are as follows: (1) Under the same conditions, the smaller tube diameter leads to the larger airside convective heat-transfer coefficient. Besides, the deviation between the Nusselt number of the experiment and the empirical correlation of Žukauskas is in the range between -14 and -10%; (2) The effect of transverse distance on heat transfer is not obvious, but the convective heat-transfer coefficient increases significantly with the increase of row number; (3) The external pressure drop of the tube exhibits an exponential increase with the air-flow rate. Particularly in the experimental samples with smaller diameters, the outflow resistance of the tube is noticeably higher compared to other tubes. Finally, new empirical correlations of the airside convection heat transfer for the small-diameter staggered tube bundles are fitted according to the experimental data, and it is hoped to provide a reference for the more accurate design of tube-bundle heat exchangers.

Heat and mass transfer of water flow in graphene nanochannels: effect of pressure and interfacial interaction.

The low-resistance transport of water within graphene nanochannels makes it promising for electronic cooling applications. However, how the water pressure and the water-graphene interaction strength affect the flow field and the thermal transfer, including velocity slip, friction coefficient, Nusselt number, temperature slip, interfacial thermal resistance, and variation of physical properties, is still not clearly understood. In this paper, we employ molecular dynamics (MD) simulations to investigate qualitatively the heat transfer of water flow in graphene nanochannels. Our results reveal that the water peak density near the wall increases approximately linearly with water pressure and water-graphene interaction strength. The water peak density near the water-graphene interface is a key factor in regulating interfacial flow and heat transfer characteristics. Under constant inlet temperature, the relationship between velocity slip length and peak density follows a consistent power function, simply modifying the pressure or the interaction strength doesn't bring a specific effect. The Nusselt number and interfacial thermal resistance are not solely dependent on water peak density; at the same water peak density, increasing interaction strength results in lower interfacial thermal resistance compared to increasing pressure. Increasing pressure improves both interfacial heat transfer and internal heat transfer of water. Furthermore, the convection heat transfer coefficient increases approximately linearly with flow resistance when pressure and interaction strength vary moderately. Finally, we notice that pressure and interaction strength hardly affect the variation range of viscosity and thermal conductivity at a channel height of 10-12 nm. These qualitative insights could lead to the development of more efficient cooling systems for electronic devices.

Thermal Analysis of Compulsator Based on Fluid-Structure Coupling and Distributed Convective Heat Transfer Coefficient

Thermal Analysis of Compulsator Based on Fluid-Structure Coupling and Distributed Convective Heat Transfer Coefficient

Evaluating the influence of convective heat transfer coefficient on the carbon footprint of a service building in Morocco

This study examines how the convective heat transfer coefficient impacts the carbon footprint of a service building situated in Morocco. This coefficient is pivotal in shaping the thermal efficiency of buildings, influencing energy consumption and environmental impact. Through simulations and analyses, we evaluate the extent to which variations in this coefficient affect overall energy efficiency and carbon emissions. Our analysis, based on specific climate data for Morocco and detailed architectural and operational parameters of a typical service building, reveals significant annual deviations. Heating energy fluctuates by up to ±48%, and cooling energy varies by up to ±32%. Furthermore, our findings demonstrate that the carbon footprint of electricity consumption for heating and cooling can vary by as much as ±31% of total CO2 emissions annually. Optimizing the convective heat transfer coefficient emerges as a critical strategy for reducing the carbon footprint, underscoring its importance in sustainable building design. These results offer valuable insights for architects, engineers, and policymakers seeking to enhance building performance and minimize environmental impact within the unique climate conditions of Morocco.

Numerical Investigation Into Convective Heat Transfer Coefficient of the Grinding Fluid Used in a Deep Grinding Process

Numerical Investigation Into Convective Heat Transfer Coefficient of the Grinding Fluid Used in a Deep Grinding Process

IMPROVEMENT OF COOLING PERFORMANCE AND MITIGATION OF FIRE PROPAGATION IN LITHIUM-ION BATTERIES USING A NOVEL GAS-COOLED THERMAL MANAGEMENT SYSTEM

Development of battery thermal management systems has become significant because an inappropriate operating temperature is the primary cause of battery deterioration, fires, and explosions. The current work proposes a revolutionary battery thermal management system that uses an inert gas instead of air as a coolant to increase cooling performance and avoid battery thermal runaway and fire propagation. The thermal behavior of 18650 cylindrical lithium-ion battery modules was assessed using a test station. The cooling performance of this battery thermal management system was investigated using Ansys Fluent, while the fire dynamics simulator assessed fire propagation when the battery was surrounded by various inert gas coolants. Inert gases can be more effective coolants than air. They were successful in lowering the maximum temperature and enhancing the convective heat transfer coefficient. Increased turbulent flow contributed in enhancement of heat transfer, as assessed by the Nusselt number. Although all gas coolants in this investigation effectively kept the cell temperature below 60°C, which is the starting temperature of thermal runaway, a high Reynolds number was necessary. Otherwise, helium is the best coolant for transferring heat from a battery, even at extremely low Reynolds numbers. When the battery was surrounded by inert gas, fire propagation in the battery can be minimized. Use of an air-cooling system can initiate fires and explosions when battery thermal runaway occurs.

Numerical Study of the Effect of the Ejection Angle on the Cooling Performance of a High-Pressure Gas Turbine

In modern gas turbines, the purge flow plays an important role in improving the performance of the gas turbine, as it is adapted against the problem of the ingestion of hot gases in the inter-disc cavity of the turbine. The performance of this purge flow is necessarily influenced by some geometrical and physical parameters. This paper presents a 3D numerical study on the influence of the ejection angle on the cooling performance in a high pressure transonic gas turbine with the presence of the cavity. Five inclination cavity angles are compared (𝝓 = 30°, 45°,60°,75°and 90°) by using ANSYS Fluent, a steady flow Reynolds average Navier-Stokes equations are solved, and the turbulence model k-ω SST has been chosen based on comparison of the distribution of velocity and heat transfer convective coefficient with experimental results in a previous work. It was seen that that cooling is limited to the suction side because of the non-uniform pressure distribution of the main flow and the high pressure of the stagnation point. The optimal configuration was determined for an ejection angle of 30°.

Simulated and experimental study on effect of thermal insulation layer on temperature field and heat dissipation of roadway surrounding rock

As mining activities extend deeper, intense heat dissipation from surrounding rock has become the most significant factor leading to heat hazards in deep mines. Laying the thermal insulation layer (TIL) along the roadway walls is an effective measure to reduce heat dissipation. This study is to investigate the temperature field and heat dissipation evolution of roadway surrounding rock after applying thermal insulation layer (TIL) by numerical simulation and similar experiment. The study shows that (i) The simulation curves agree well with the experimental data, which confirms the reliability of the mathematical model and numerical simulator; (ii) The application of TIL slows down the cooling of surrounding rock temperature field, the cooling ring radius decreased by 8.13 % after 10 years of ventilation; (iii) The effect of TIL on the equivalent thermal resistance (ETR) is most significant in early ventilation, and the thermal insulation effect decreases with time; (iv) The heat insulation rate is positively correlated with the TIL thickness and the convective heat transfer coefficient, and negatively with the thermal conductivity of TIL. This study provides the theoretical basis for the use of thermal insulation materials in underground mines.

Experimental and numerical study on the hydrothermal performances of a mirror Y-shaped convergent mini-channel heat sink

To enhance the hydrothermal performances of conventional mini-channel heat sinks, this paper experimentally and numerically investigates the impacts of channel convergence α on the pressure drop (ΔP), the maximum temperature (Tmax), the convective heat transfer coefficient (hs), and the coefficient of performance (COP) of a mirror Y-shaped convergent mini-channel heat sink (MYCMCHS) with a rectangular channel cross-section when the overall channel volume is fixed. The results show that ΔP of MYCMCHS initially decreases and subsequently increases with the growth of the channel convergence α; however, Tmax declines but hs grows with the drop of α. The different dependences of hydraulic and thermal performances on the channel convergence result in the COP of the MYCMCHS exhibiting an initial increase followed by a subsequent decrease with the rise of α. The numerical results further indicate that at the inlet volume flow rates of 0.05, 0.1, and 0.15 L/min, the maximum COP achieved at α = 0.7 rises 19.91 %, 20.57 %, and 22.59 % respectively in comparison to the COP of the heat sink without channel convergence (α = 1). This work validates the performance advantages of channel convergence and provides a possibility to enhance the hydrothermal performances of heat sink.

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.

Assessment of safety and economic impact of boil-off-gas in LPG storage tanks

This study was carried out to assess the effect of boil-off gas (BOG) on the safety and economic effectiveness of LPG storage tanks. It includes analysis of the thermodynamic properties of LPG; heat absorbed from ambient air by the storage tank that leaks into the LPG, and consequently generates boil-off gas in the supply chain, by utilizing appropriate thermodynamic and heat transfer equations. Analyzing the heat leakage required the estimations of the convective heat transfer coefficient of the ambient air in the supply location and the LPG supply chain, which amounted to 3335.9W/m2K and 21.058W/m2K, respectively, in the system under study. Analyzing the thermodynamic properties, such as specific volume, entropy, and enthalpy of the LPG, shows that the entropy of LPG in the storage tank is negative, which suggests an endothermic process, validating that heat is added to the system from the surroundings. The heat absorbed in the LPG from the ambient air by the storage tank amounted to 1.785kW. The boil-off generation rate due to the storage tank heat leakage was 0.0049kg/s, which translates to a cost equivalent loss of 0.0069$/s at an LPG selling price of 1.42$/kg. It was recommended that maintenance of insulation and other external factors such as wind speed, solar radiation, ambient temperature, and thermal conductivity of the storage tank material are key factors in minimizing the heat leaks into LPG; hence BOG generation, which is of utmost importance in ensuring safety and economic loss in the LPG supply chain.