Papers by Keyword: Convection

Abstract: This work numerically studies the thermal management of a Li-ion battery pack using Phase Change Materials (PCMs) with two different modelling approaches. Specifically, the results obtained with the Enthalpy-Porosity method, implemented in the tool STAR-CCM+, are compared with those yielded by the Apparent Heat Capacity formulation, employed by COMSOL Multiphysics. Both models are first validated against benchmark cases found in the literature. The study then focuses on the thermal behaviour of a battery pack composed of four 21700 Li-ion battery cells, cooled using the paraffinic PCM RT35. The numerical results show that, while natural convection in the liquid PCM accelerates the melting process, it leads to a non-uniform temperature distribution, particularly disadvantageous for cells located in the upper part of the battery pack. In addition, although both numerical approaches show good agreement between their results, especially in capturing the overall thermal behaviour, some minor differences in the temperature profiles during the PCM phase change still emerge.

Abstract: Firefighter protective clothing is a crucial piece of equipment for firefighters during firefighting operations. However, the current firefighting clothing, when exposed to high temperatures around 100°C, provide only about 10 minutes of protection, which is insufficient. To enhance the protective performance of firefighter clothing, the most effective approach is to incorporate a cooling source, which can then transfer its cooling energy to the suit via a circulating medium, helping to regulate the body's temperature. Water is the most commonly used circulating medium, but it significantly increases the weight of the clothing. To address the issue of balancing the weight of the cooling system and its protective effectiveness, this paper proposes using air inside the firefighting clothing as the circulating medium. This would enhance the internal heat transfer through convection. In this study, a seven-layer geometric model is constructed using finite element software. The model includes the external air layer, outer layer, waterproof and breathable layer, thermal insulation layer, comfort layer, internal air layer, and skin layer. The temperature distribution and changes on the outer surface, inner surface, and human body surface of the suit are analyzed. The material of the firefighting clothing is modeled as a porous medium, while organic silica gel is used to simulate human skin. A wet air convection heat transfer model is developed to assess its thermal protection performance. The model's reliability is verified through experimental validation. The model is then used to examine the impact of external air temperature and internal air layer thickness on the thermal protection performance of the firefighting clothing. It was found that the internal air layer significantly influenced the thermal protection: The thermal protection of the suits with air convection was significantly improved compared to the thermal protection of the suits without air convection. when the external temperature increased from 50°C to 100°C, the surface temperature of the human body rose by only 2.24°C. However, when the internal air layer thickness was reduced from 10 mm to 2 mm, the human body surface temperature increased by 4.21°C, and thermal comfort decreased, though it still did not exceed the thermal safety limit.

Abstract: To overcome the inlet temperature uncertainties during an in-line thermo-rheological characterization and to further apply a differential convection method for an injection molding process, a concept of device designing is proposed in this work. An analytical and numerical investigation proves that the proposed concept can provide information on the viscosity of the material via thermal measurements, despite a poorly known inlet temperature.

Abstract: This study investigates transport process in circular tubes cross-flow Heat Exchanger (HEX) using water-CuO-nanofluids cooling media. The effects of nanoparticle volume fractions (Ø) and Reynolds number (Re) on the flow structure, coefficient of skin friction, isotherms and Nusselt number (Nu) are determined for steady laminar flow. The governing equations of continuity, momentum and energy are discretized over the flow domain and solved using SIMPLE method of the Finite Volume Method with ANSYS Fluent 16. The results show that the flow field for the conventional fluid is concentric around the inner tubes for Re up to 60 after which vortices evolve downstream behind the tubes, elongate and eclipse with the increase in Re. Vortex inception occurs at Re between 60 and 45 for 0 ≤ Ø ≤ 10%. The temperature fields are characterized by plume-like structure which envelopes the two inner cylinders between which heat transfer occurs. The average Nusselt number is correlated as Nu = 22.4 - 411,588س + 0.757Re + 1803.31/ln(Re) in which the interaction between Re and Nu has significant (p ≤ 0.05) effect. The addition of nanoparticles in the range 2 ≤ Ø ≤ 10% results in the increase in Nu from 0.55 to 5.84%. It follows that the thermal performance of the cross flow heat exchanger could be enhanced with CuO-based nanofluids.

Abstract: In order to predict the effect of the Marangoni convection and the morphology of melted stainless steel powder, during the selective laser melting (SLM) process, a transient three-dimensional numerical model is developed at the mesoscale. The evolution of the temperature and velocity fields’ is then studied. The initial powder bed distribution is obtained by the discrete element method (DEM) calculation, and the temperature distribution and the molten pool shape deformation are calculated and analyzed by the Ansys-Fluent commercial code. The molten pool shape is obtained by considering the influence of Marangoni convection on the internal flow behavior. The recoil force was not considered in our calculation. As main results, a slight deviation between the position of the maximum temperature of the molten pool and the center of the laser spot is observed. The direction of the heat diffusion is more likely to be horizontal and the flow centrifugal, which causes the melt track to be wide. Finally, the Marangoni convection is the main driver of the flow.

Abstract: The occurrence of flow pattern can be predicted based on constructal law. Scale analysis is a method for deriving the essential information based on the basic principles of fluid flow and heat transfer. It provides order-of-magnitudes but also the form of the functions that describe the quantities understudy. In flow systems, patterns (configuration, design, architecture) arise from competition between competing trends, at least two modes of transport or locomotion: slow (diffusion, walk, etc.) and fast (streams, run, etc.). Optimal patterns mean the best flow access and the best balance between these trends. The study presented here follows from the scale analysis together with constructal and, is illustrated by examples from simple water heating to human locomotion.

Abstract: Single pellet heating was considered at natural gas combustion product movement with oxidant flow coefficient of α=1.0 with air and oxygen in unconstrained volume. Physical parameters (density, dynamic and kinematic viscosity) and heat transfer properties (temperature, heat capacity, thermal conductivity, thermal diffusivity) of combustion products correspond to the average composition of gas delivered to the Ural region. Iron-ore pellet average properties were evaluated, based on major pellet plant data. Convection and radiant heat transfer coefficients were determined at pellet heating in natural gas combustion products in a mixture with air and oxygen. It was noted that, at switching to gas burning with oxygen radiant heat transfer to the pellet surface increases significantly, as compared to the convection one.

Abstract: Drying characteristics of watermelon seeds using infrared (IR) heating combined to non-heated air flow were determined varying the IR source temperature and air velocity. The effects of the process variables on the effective moisture diffusivity (D_eff) and specific energy consumption (SEC) were also evaluated. Experiments in the hybrid dryer were conducted with seeds arranged in a single layer and exposed to three IR temperatures levels (45, 65 and 85 W/m²) and three air velocities levels (0.4, 0.8 and 1.2 m/s) at 25°C. The effective moisture diffusivity was estimated using Fick’s diffusion model assuming negligible shrinkage and surface moisture in equilibrium with the surrounding air. D_eff-values ranged from 0.62 x 10^-10 to 1.83 x 10^-10 m²/s, while SEC-valued varied from 29.91 to 73.16 kWh/g. Statistical analysis carried out on the experimental data indicated that the effective moisture diffusivity and specific energy consumption were significantly influenced only by the IR source temperature, which had a positive linear effect on D_eff and a negative linear effect on SEC. Maximum effective diffusivity and minimum energy consumption values in hybrid drying of watermelon seeds were obtained with the use of the highest IR temperature and lowest air velocity.

Abstract: The performance of carbon nanotube (CNT) nanofluids on convective heat transfer over a stretching sheet was investigated under thermal stratification and magnetic field effects. Water, engine oil and ethylene glycol are used as the base fluids. The governing equations are transformed into a system of coupled nonlinear ordinary differential equations using similarity transformations and solved numerically using the fourth-order Runge–Kutta–Fehlberg in conjunction to shooting method. The CNT nanofluids with an engine oil base fluid shows the highest thermal conductivity in comparison to ethylene glycol and water, respectively. Potential application of the thermal conductivity enhancement of CNT nanofluid is to increase the energy-efficient mechanical systems in heating, cooling and ventilation of the indoor environment.

Abstract: The present study deals with the numerically investigation of developing laminar natural convection in the vertical double-passage porous annuli formed by three vertical concentric cylinders of which the middle cylinder is a thin and perfectly conductive known as baffle. In this analysis, two thermal conditions are considered namely, either inner or outer cylindrical wall is constantly heated while the opposite wall is insulated. An implicit finite difference technique is employed to solve the boundary layer equations in both the annular passages. The temperature profiles and velocity profiles in axial as well as radial directions have been presented for different values of Grashof number, Darcy number, baffle position and radius ratio. The results reveal that both physical and geometrical parameters have profound influence on the development of velocity and thermal fields as well as heat transfer rate.