Evaluating the Effect of Cover Materials on Greenhouse Microclimates and Thermal Performance

This study compared and analyzed changes in the microclimate and thermal environment inside single-span greenhouses covered with a single layer of plastic film, polycarbonate (PC), and glass. The results of the experiment show that the PC-covered greenhouse was the most favorable for managing the nighttime heating effect during the cold season. However, the glass-covered greenhouse was found to be the most favorable for managing the cooling effect during the hot season. Although the plastic-covered greenhouse was inexpensive and easy to install, the air temperature inside varied significantly, and it was difficult to control its indoor environment. The thermal load leveling values showed that the PC-covered greenhouse had the lowest variation, confirming its superiority in terms of environmental control and energy savings. In terms of the overall heat transfer, heat was generally transferred from the interior to the exterior of the greenhouses. In the plastic-covered greenhouse, however, heat was transferred in the opposite direction at night due to the influence of radiant cooling. The occurrence of the minimum and maximum heat transfer values had a tendency similar to that of the occurrence of the minimum and maximum air temperatures inside the greenhouses.

Comparative Study of Thermal Performance of Different Nanofluids in a Double Backward-Facing Step Channel: A Numerical Approach

Two-dimensional numerical simulations are conducted to study forced convection flow of different water-based nanofluids (ZnO, Al2O3, and SiO2) with volume fractions ( ϕ ) = 0–5% and fixed nanoparticle size (dp) = 20 nm for Reynolds numbers (Re) = 50–225 over a double backward-facing step with an expansion ratio (ER) = 2 under constant heat flux (q″ = 3000 W/m2) condition using the finite volume method. Results indicate that the local Nusselt number increases with volume fraction and Reynolds number for all working fluids. In comparison to water, the maximum heat transfer augmentation of about 21.22% was achieved by using water-SiO2 nanofluid at Re = 225 with ϕ = 5% and dp = 20 nm. Under similar conditions, the Al2O3 and ZnO nanofluids demonstrated 14.23% and 11.86% augmentation in heat transfer in comparison to water. The skin friction coefficient decreases with the increase in Re for all working fluids. No significant differences are observed in the values of skin friction coefficient among all working fluids at a particular Re. These results indicate that the heat transfer enhancement has been achieved with no increased energy requirements. In addition, the velocity increases with the rise in Re, with SiO2 nanofluid exhibiting the highest velocity as compared to other working fluids.

Experimental and Numerical Investigation on Thermal Damage of Granite Subjected to Heating and Cooling

Rock mass is frequently subjected to rapid cooling in geothermal reservoir during water injection and reinjection. In this paper, to understand the effects of cooling treatments on heated granite, heat conduction tests, magnetic resonance imaging tests and numerical investigations were carried out to evaluate variations of thermal damage. The test results reveal that the heat flux and the heat transfer coefficient increases to a maximum within a few seconds and then gradually decreases. The maximum heat transfer coefficient of the samples treated with the initial temperature of 500, 400, 300, 200 and 100 °C is 2.3, 2.15, 1.9, 1.22 and 1.86 W·m−2K−1, respectively. The edge area with drastic temperature changes is accompanied by the densely distributed microcracks; in contrast, the internal cracks of the specimen with gentle temperature are relatively sparse. The thermal damage contributed by the heating cracks occurs at a continuous decrease, and the thermal damage contributed by cooling occurs at a continuous increase, with the increasing heating temperature. The damage caused by heating is the result of the uneven thermal expansion of the local particles, the propagation of cooling cracks is strongly affected by heating cracks, and stress concentration induced by thermal shock promotes the coalescence of the pre-existing heating cracks.

Moist Air Flow Analysis in an Open Enclosure. Part A: Parametric Study

Heat and mass transfer in many systems are widely accomplished applying natural convection process due to their low cost, reliability, and easy support. Typical applications include different mechanisms in various fields such as (solar energy, solar distiller, stream cooling, etc…). Numerical results of turbulent natural convection and mass transfer in an open enclosure for different aspect ratios (AR = 0.5, 1, and 2) with a humid-air are carried out. Mass fraction and local Nusselt number were proposed to investigate the heat and mass transfer. A heat flux boundary conditions were subjected to the lateral walls and the bottom one make as an adiabatic wall, while the top area was proposed as a free surface. Effect of Rayleigh numbers (106≤????????≤108) on natural convection and mass flow behavior are analyzed. The governing equations are solved using CFD Fluent code based on the SIMPLE algorithm. The results showed that the cavity with an aspect ratio of AR = 2 has a significant enhancement to raise the rates of both heat and mass transfer. When the Rayleigh number increases, maximum heat transfer rates were observed due to the fluid flow becomes more vigorous. However, mass transfer improves as the Rayleigh number decreases.

Numerical study of gas dynamics and heat transfer in matrix channels at various rib angles

The results of numerical simulation of the separation flow in matrix channels by the RANS method are presented. The simulation is performed at the Reynolds number Re = 12600, determined by the mass-average velocity and the height of the channel. The distribution of the local Nusselt number is obtained for various Reynolds numbers in the range of 5÷15⋅103 and several rib angles. It is shown that the temperature distribution on the surface is highly nonuniform; in particular, the maximum heat transfer value is observed near the upper edge facets, in the vicinity of which the greatest velocity gradient is observed.

Heat transfer characterization under radial jet and falling film induced rewetting

Empirical correlations are developed for rewetting velocity and maximum heat transfer coefficient during rewetting phase of single hot vertical Fuel Pin Simulator (FPS) by using radial jet impingement and falling film. Emergency Core Cooling System (ECCS) has been designed for Advance Heavy water Reactor (AHWR) to rewet the hot fuel pin under the loss of coolant accident. Coolant injection takes place from a water rod which is located at the center of the fuel bundle in form of jets to rewet hot surface of fuel pin under loss of coolant accident. This kind of design to reflood the fuel bundle is different than bottom and top spray reflooding practiced in PWR and BWR type of nuclear reactors. There are two different kinds of rewetting found during radial jet induced cooling. The first one is due to radial jet impingement and the second one is due to falling film which is below the jet impingement point. Rewetting velocity has been predicted along the length of fuel pin due to radial jet impingement cooling. Temperature of FPS has been varied from 400°C to 700°C with help of different powers supply, simulating decay heat of reactor. A variation of coolant radial jet mass flow rate is from 0.5 lpm to 1.8 lpm. It is considered during ECCS injection. It has been observed from the experiments that rewetting velocity decreases with increasing the clad surface temperature and increases with increasing the coolant mass flow rate. The rewetting velocity in falling film is found to be nearly 1.8 times higher than rewetting velocity predicted in circumferential direction. Further, it is found that maximum heat transfer coefficient increases with increasing the radial jet coolant mass flow rate. The maximum heat transfer coefficient in case of radial jet impingement is found to be nearly 1.5 times the falling film rewetting. Developed correlation predicts the maximum heat transfer coefficient with experimental data well within the error band of ±10%.

Computational Fluid Dynamics of Heat Transfer in Stirred Tank Reactors

In the present work, computational fluid dynamics study of stirred tanks of three sizes (20L, 400L and 5000L) provided with helical coils has been carried out. Various design parameters (impeller diameter, type and clearance) and operational parameters (Reynolds Number and Power per unit volume) have been varied and their effect on process side heat transfer coefficient has been studied. CFD model is validated with experimental work of Cummings and West[9] and in house experimentation. Design settings of D/T=0.5, C/T=0.33 for PBTD450 resulted in maximum heat transfer (5440 W/m2K for P/V=1000 W/m3). For constant RPM and constant D/T (Constant Reynolds Number), Increasing the power number of impeller increased process side HTC at the cost of increased power requirement (decreasing efficiency). In such cases, proper selection of impeller system needs to be made based on the requirements of heat removal and optimizing parameters such as product yield, product quality etc.

Guidelines for Designing Micropillar Structures for Enhanced Evaporative Heat Transfer

Over the last several decades, cooling technologies have been developed to address the growing thermal challenges associated with high-powered electronics. However, within the next several years, the heat generated by these devices is predicted to exceed 1 kW/cm2, and traditional methods, such as air cooling, are limited in their capacities to dissipate such high heat fluxes. In contrast, two-phase cooling methods, such as microdroplet evaporation, are very promising due to the large latent heat of vaporization associated with the phase change process. Previous studies have shown non-axisymmetric droplets exhibit different evaporation characteristics than spherical droplets. For a droplet pinned atop a micropillar, the solid-liquid and liquid-vapor interfacial area, the volume, and thickness of the droplet are the major factors that govern the evaporation heat transport process. In this work, we develop a shape optimization tool using the particle swarm optimization algorithm to maximize evaporation from a droplet confined atop a micropillar. The tool is used to optimize the shape of a nonaxisymmetric droplet. Compared to droplets atop circular and regular equilateral triangular micropillar structures, we find that droplets confined on pseudo-triangular micropillar structures have 23.7% and 5.7% higher heat transfer coefficients, respectively. The results of this work will advance the design of microstructures that support droplets with maximum heat transfer performance.

Stratified Shear-Thinning Fluid Flow Past Tandem Cylinders in the Presence of Mixed Convection Heat Transfer with a Channel-Confined Configuration

The article examines the consequence of thermal buoyancy-driven cross-flow and heat transfer for shear-thinning power-law fluids on the tandem orientation of two cylinders. Finite volume methodology is used to investigate the effect of the gap ratio (2.5 ≤ S/D ≤ 5.5), power-law index (0.2 ≤ n ≤ 1) and Richardson number (0 ≤ Ri ≤ 1) on flow and thermal output parameters at Reynolds number Re ≤ 100 and Prandtl number Pr ≤ 50 in a confined channel. An unprecedented jump has been witnessed in the flow/thermal parameters at the critical gap ratio (critical spacing). At forced convection (Ri ≤ 0), this critical spacing keeps on increasing with shear-thinning character, from S/D = 3.9 (at n = 1) to 4.9 (at n = 0.2). On the contrary, an increase in shear-thinning characteristic leads to a decrease in critical spacing from S/D = 3.9 (at n = 1) to 2.8 (at n = 0.4) for Ri = 1 (mixed convection). The heat transfer rate increases with shear-thinning behavior, with a maximum heat transfer, noted at n = 0.2. A higher unprecedented increment for flow/thermal parameters is seen at critical spacing for the downstream cylinder than the upstream cylinder. At the highest gap ratio, the output parameters for the upstream cylinder approximate that of an isolated cylinder. The time-variant fluctuations in lift coefficients for a shear-thinning flow in a tandem arrangement provide a new understanding of co-shedding and extended body flow regimes.

Effect of liquid cooling on PCR performance with the parametric study of cross-section shapes of microchannels

In recent years, PCR-based methods as a rapid and high accurate technique in the industry and medical fields have been expanded rapidly. Where we are faced with the COVID-19 pandemic, the necessity of a rapid diagnosis has felt more than ever. In the current interdisciplinary study, we have proposed, developed, and characterized a state-of-the-art liquid cooling design to accelerate the PCR procedure. A numerical simulation approach is utilized to evaluate 15 different cross-sections of the microchannel heat sink and select the best shape to achieve this goal. Also, crucial heat sink parameters are characterized, e.g., heat transfer coefficient, pressure drop, performance evaluation criteria, and fluid flow. The achieved result showed that the circular cross-section is the most efficient shape for the microchannel heat sink, which has a maximum heat transfer enhancement of 25% compared to the square shape at the Reynolds number of 1150. In the next phase of the study, the circular cross-section microchannel is located below the PCR device to evaluate the cooling rate of the PCR. Also, the results demonstrate that it takes 16.5 s to cool saliva samples in the PCR well, which saves up to 157.5 s for the whole amplification procedure compared to the conventional air fans. Another advantage of using the microchannel heat sink is that it takes up a little space compared to other common cooling methods.