Search Results (36)

Morocco, like many nations undergoing significant economic and social transformation, is experiencing rapid urbanization alongside an ongoing rural exodus. This, coupled with the country’s diverse climate and heterogeneous geography, warrants a detailed exploration of urbanization’s effect on surface climate. Utilizing the Simple Biosphere (SiB2) model’s simulated surface temperature, this study analyses summer’s urban heat structure of seven Moroccan urban areas and their surroundings, assessing the urban impact on surface temperature at the city center, and the intensity and spatial distribution of the urban heat island (UHI) effect at different spatial resolutions. Results show wide-ranging dissimilarities in urban thermal profiles, with the maximum UHI intensity recorded at 8.7 °C in the Dakhla peninsula. Urban heat sink (UHS) effects were observed in six of the seven studied cities, with Marrakech being the exception, only exhibiting UHI effects. A more detailed examination of the thermal profile in Rabat’s metropole at a finer scale, using Landsat-observed land surface temperature (LST), yields additional insights into UHI characteristics, and the findings are contrasted with the existing literature to provide broader insights. The implications of this study strongly resonate within the Moroccan context and its neighboring regions with similar environmental and socio-economic features and should aid in the development of sustainable regional urban planning. Full article

The global development of LED lighting in all applications for both public and indoor lighting establishes a very important lever for reducing the carbon impact by significantly reducing energy consumption. Smart lighting will therefore constitute an essential lever in the smart city of tomorrow. The latter is more sustainable and less energy-intensive than other light sources, contributing towards the Sustainable Development Goals set forth by the United Nations (SDGs 7 and 11). With its ease of integration, ergonomics, lightness, and high quality of light, this innovative light source has two major qualities: its energy efficiency and its long service life. However, poor thermal management has catastrophic effects on these two essential criteria. This necessarily requires optimizing thermal management and heat sinks. In some cases, thermal models and simulations can lead to considerable calculation times because they implement many parameters. This work therefore aims to reduce the number of these simulations by applying the method of experimental designs (Taguchi) and analysis of variance (ANOVA) to simulations intended to optimize the dissipation of LED luminaires. We applied the method to a simple finned heat sink model for a unit input power and then to a concrete case of a high-power LED. The control parameters and their respective contributions in the proposed model were studied. The ANOVA results corroborated the conclusions of the Taguchi method, demonstrating a strong agreement between these analytical methods, in which the temperature was adjusted by identifying optimal combinations of parameters. The fields of application relevant to this study include both indoor lighting (false ceiling) and confined spaces with severe sealing constraints such as car headlights or the optical blocks of urban luminaires. Full article

Over the past two decades, double-layered microchannel heat sinks (DL-MCHs) have become increasingly popular as they provide effective performance for electronic cooling, particularly in the counterflow configuration. The cross-flow configuration, which requires much simpler headers, has seldom been considered in the scientific literature, probably due to the possible formation of a hotspot near the outlet port. The aim of this study is to show that cross-flow DL-MCHs can provide performance levels that are comparable to those attained by counterflow DL-MCHs by exploiting the nonuniform flow distribution produced by properly designed headers. Numerical simulations are performed using in-house finite element procedures to solve the parabolized Navier–Stokes equations in the microchannels and the energy equation in the entire computational domain. The analysis is carried out both for ideal linear microchannel velocity distributions and for the realistic velocity distributions induced by headers with or without baffles, as proposed by the authors in previous papers. The optimal degree of velocity nonuniformity in the microchannels yielding the best thermal performance was found to depend on the flow rate. For instance, in the case of optimal linear variations of the microchannel velocity distribution, the thermal resistance was reduced by 11.8%, 7.1%, and 4.4% compared to the case with uniform inlet velocities, and it was only 3.4%, 1.8%, and 0.3% higher than that of the counterflow configuration for average microchannel velocities equal to 0.5, 1, and 2 m/s, respectively. The main conclusion is that the cross-flow configuration, with its simple headers and piping, can achieve thermal resistance and temperature uniformity on the heated surface that are very similar to that of the counter-flow configuration through proper header design that ensures a suitable microchannel velocity distribution. Full article

Several theoretical studies have predicted that refrigerant mixtures with glides of more than 20 K can yield COP improvements in heat pumps for operating conditions where the temperature difference between the heat source and heat sink is large, but experimental validations and quantifications are scarce. The application of high-glide mixtures (>20 K) in industrial heat pumps in the field is, therefore, still hampered by concerns about the behavior and handling of the mixtures. This study experimentally investigates hydrocarbon (HC) mixtures R-290/600 (propane/butane) and R-290/601 (propane/pentane) and compares them to previously tested mixtures of synthetic refrigerants. Comprehensive evaluations are presented regarding COP, compressor performance, pressure drop, heat transfer, and the possibility of inline composition determination. The mixtures were tested over a range of compositions at a source inlet temperature of 60 °C and a sink outlet temperature of 100 °C, with the heat sink and heat source temperature differences controlled to 35 K. R-290/601 at a mass composition of 70%/30% was found as the best mixture with a COP improvement of 19% over R-600 as the best pure fluid. The overall isentropic compressor efficiency was similar for HC and synthetic refrigerants, given equal suction and discharge pressures. Pressure drops in heat exchangers and connecting lines were equal for synthetic and HC mixtures at equal mass flow rates. This allows higher heating capacities of HC mixtures at a given pressure drop (mass flow rate) due to their wider vapor dome. A previously developed evaporator heat transfer correlation for synthetic refrigerant mixtures was applicable to the HC mixtures. A condenser heat transfer correlation previously fitted for synthetic refrigerants performed significantly worse for HC mixtures. Composition determination during operation and without sampling was possible with a deviation of at most 0.05 mass fraction using simple temperature and pressure measurements and REFPROP for thermodynamic property calculations. Overall, high-glide HC mixtures, just like mixtures of synthetic refrigerants, showed significant COP improvements for specific operating conditions despite a decreased heat transfer coefficient. Potential problems like composition shift or poor compressor performance were not encountered. As a next step, testing high-glide mixtures in pilot-plant installations is recommended. Full article

Conventional liquid cooling techniques may provide effective chip cooling but at the expense of high pumping power consumption. Considering that there is dynamic heat load in practice, a self-adaptive cooling technique is desired to reduce operational costs while preserving inherent cooling effectiveness. In this work, a novel self-adaptive cooling strategy is presented to balance the thermal and flow efficiency in accordance with the dynamic thermal load, based on temperature-regulated movement of the metal pillar array in a microfluidic channel. With an illustrative device, the effectiveness of such a strategy is investigated using multiphysics modeling and simulation. As a case study, the device is considered to be initiated with a chip power of 5 W and an inlet coolant velocity of 0.3 m/s. It is shown that the temperature-regulated movement of the metal pillar heat sink will be activated rapidly and equilibrate within 30 s. Parts of the metal pillars immerse into the coolant flow, resulting in significantly improved heat transfer efficiency. The diminished thermal resistance leads to a reduction in chip temperature rise from 225 K (without structural adaptation) to 91.86 K (with structural adaption). Meanwhile, the immersion of metal pillars into the coolant also causes an increased flow resistance in the microfluidic channel (i.e., pressure drop increases from 859.27 Pa to 915.98 Pa). Nevertheless, the flow resistance decreases spontaneously when the working power of the chip decreases. Comprehensive simulation has demonstrated that the temperature-regulated structure works well under various conditions. Therefore, it is believed that the presented self-adaptive cooling strategy offers simple and cost-effective thermal management for modern electronics with dynamic heat fluxes. Full article

Wildfires spread rapidly and cause considerable ecological and socioeconomic losses. Inner Mongolia is among the regions in China that suffer the most from wildfires. A simple, effective model that uses fewer parameters to simulate wildfire spread is crucial for rapid decision-making. This study presents a region-specific technological process that requires a few meteorological parameters and limited grassland vegetation data to predict fire spreading dynamics in Inner Mongolia, based on cellular automata that emphasize the numeric evaluation of both heat sinks and sources. The proposed method considers a case that occurred in 2021 near the East Ujimqin Banner border between China and Mongolia. Three hypothetical grassland wildfires were developed using GIS technology to test and demonstrate the proposed model. The simulation results suggest that the model agrees well with real-world experience and can facilitate real-time decision-making to enhance the effectiveness of firefighting, fire control, and simulation-based training for firefighters. Full article

Significant circulating currents in the modular multilevel converter (MMC) increase system losses and complicate heat-sink design. Conventional PI and PR controllers can achieve steady-state error adjustment, but are sensitive to parameter changes and model uncertainty, heavily relying on coordinate transformations and careful design of model parameters. Model predictive control (MPC) has the characteristics of simple design, good robustness, and excellent dynamic response; however, it encountered the complexity of adjusting weighting factors. This paper proposed circulating the current model predictive proportional integral control (MPPIC) method in abc reference frame. This hybrid control solution utilized the predictive model and traditional PI algorithm to combine the advantages of nonlinear and linear control. Compared with existing suppression methods, this method avoided complex mathematical operations and a selection of weight coefficients, was easy to implement, and can effectively suppress circulating currents under different modulation ratios. Simulations were conducted on MATLAB/Simulink to verify the effectiveness of the proposed control strategy. MPPIC can not only distinctly suppress the circulating currents, but also reduce the overall voltage fluctuation of sub-modules capacitors under different modulation ratios, and had almost no any adverse effect on the performance of MMC. Full article

Surface turbulent heat fluxes are crucial for monitoring drought, heat waves, urban heat islands, agricultural water management, and other hydrological applications. Energy Balance Models (EBMs) are widely used to simulate surface heat fluxes from a combination of remote sensing-derived variables and meteorological data. Single-source EBMs, in particular, are preferred in mapping surface turbulent heat fluxes due to their relative simplicity. However, most single-source EBMs suffer from uncertainties inherent to the parameter $k{B}^{-1}$, which is used to account for differences in the source of heat and the sink of momentum when representing aerodynamic resistance in single-source EBMs. For instance, the parameterization of $k{B}^{-1}$ in the commonly used single-source Surface Energy Balance System (SEBS) model uses a constant value of the foliage heat transfer coefficient ($C_t$), in the parameterization of the vegetation component of $k{B}^{-1}$ ($k{B}_v{}^{-1}$). Thus, SEBS ignores the effect of turbulence on canopy heat transfer. As a result, SEBS has been found to greatly underestimate sensible heat flux in tall forest canopies, where turbulence is a key contributor to canopy heat transfer. This study presents a revised parameterization of $k{B}_v{}^{-1}$ for the SEBS model. A physically based formulation of $C_t$, which considers the effect of turbulence on $C_t$, is used in deriving the revised parameterization. Simulation results across 15 eddy covariance (EC) flux tower sites show that the revised parameterization significantly reduces the underestimation of sensible heat flux compared to the original parameterization under tall forest canopies. The revised parameterization is relatively simple and does not require additional information on canopy structure compared to some more complex parameterizations proposed in the literature. As such, the revised parameterization is suitable for mapping surface turbulent heat fluxes, especially under tall forest canopies. Full article

Electrical insulation failure is the most common failure mechanism in electrical machines (motors and generators). High temperatures and/or temperature gradients (HTTG) are the main drivers of insulation failure in electrical machines. HTTG combine with and augment other destructive effects from over-voltage, to voltage transients, overload and load variations, poor construction techniques, and thermal cycling. These operating conditions cause insulation damage that leads to electrical insulation failure. The insulation failure process is greatly accelerated by pollutants and moisture absorption. A simple and robust way to reduce HTTG and moisture adsorption is by maintaining constant internal temperatures. The current method to maintain elevated internal temperatures and reduce condensation issues is by internal electrical heating elements. This paper examines the effectiveness of applying thermoelectric coolers (TECs), solid-state heat pumps (Peltier devices), as heaters to raise a motor’s internal temperature by pumping heat into the motor core rather than heating the internal air. TEC technology is relatively new, and the application of TECs to heat a motor’s internal volume has not previously been explored. In this paper, we explore the hypothesis that TECs can pump heat into a motor when out of service, reducing the HTTG by maintaining high winding slot temperatures and eliminating condensation issues. This paper describes a test motor setup with simple resistive heating (traditional method), compared with the application of TECs with heat sinks, heat pipes, and a water circulation heat exchanger, to gauge the capability of TECs to heat the inner core or winding area. In this paper, we demonstrate the full integration of TECs into a motor. The results show that each of the systems incorporating the TECs would effectively pump heat into the core and keep the winding hot, eliminating condensation issues and water ingress due to thermal cycling. Full article

Even though two-phase heat transfer of refrigerants in minichannel heat sinks has been studied extensively, there is still a demand for improvements in overall thermal performance of miniature heat transfer exchangers. Experimental investigation and sophisticated heat transfer calculations with respect to heat transfer devices are still needed. In this work, a time-dependent experimental study of subcooled boiling was carried out for FC-72 flow in a heat sink, comprising of five asymmetrically heated minichannels. The heater surface temperature was continuously monitored by an infrared camera. The boiling heat transfer characteristics were investigated and the effect of the mass flow rate on the heat transfer coefficient was studied. In order to solve the heat transfer problem related to time-dependent flow boiling, two numerical methods, based on the FEM were applied, and based on the Trefftz functions (FEMT) and using the ADINA program. The results achieved with these two calculation methods were explored with an emphasis on the impact of the mass flow rate (range from 5 to 55 kg/h) on the resulting heat transfer coefficient. It was found that, with increasing mass flow, the heat transfer coefficient increased. Good agreement was found between the heat transfer coefficients, determined according to two numerical methods and the simple 1D calculation method. Full article

In the modern digital world, electronic devices are being widely employed for various applications where thermal performance represents a significant technical challenge due to continued miniaturization, high heat generated in the system, and non-uniform high-temperature causing failure. Phase change materials (PCMs) owing to the immense heat of fusion are primarily considered for thermal management, but their insulating properties hedge their applications in electronics cooling. Nano-enhanced phase change materials (NePCMs) have the ability to improve the thermal conductivity of PCM, decrease system temperature and escalate the operating time of devices. Accordingly, the current study focused on the experimental investigations for the thermal performance of three heat sinks (HS) with different configurations such as a simple heat sink (SHS), a square pin-fins heat sink (S_pfHS), and Cu foam integrated heat sink (Cu_fmHS) with various alumina nanoparticles mass concentrations (0.15, 0.20 and 0.25 wt%) incorporated in PCM (RT-54HC) and at heat flux (0.98–2.94 kW/m²). All HSs reduced the base temperature with the insertion of NePCM compared to the empty SHS. The experimental results identified that the thermal performance of Cu_fmHS was found to be superior in reducing base temperature and enhancing working time at two different setpoint temperatures (SPTs). The maximum drop in base temperature was 36.95%, and a 288% maximum working time enhancement was observed for Cu_fmHS. Therefore, NePCMs are highly recommended for the thermal management of the electronic cooling system. Full article

This study consists of a modern and economic way to recover the heat wasted from the industrial chimneys which are cooled down by natural sources by using thermal electric generator heat sinks. A flap with altered heat sink (which is attached at the top of thermo-electric generator) is used in place of convectional heat sink base. A 3D model is proposed by using AUTODESK Fusion 360 and is solved by using Workbench 2021 R1 ANSYS. The presented setup fully describes the transfer of heat along the one vertical bar inside the TEGs module which is mounted along the vertical wall of the Chimney. The impact of Flap dimensions (Height, Depth, and Angle) and conductive material performance is studied. The flap angles are 45°, 50° and 60° and depths of the flap are 28 mm, 30 mm and 33 mm. This altered heat sink accomplishes about 28% enhance in the rate of cooling of TEGs module. The maximum output power, i.e., 105 mV of the TEGs module is at 60° and at 33 mm depth. The results show that remodified heat sink maintains the system simple and requires less maintenance and also improves the cooling rate of the thermoelectric generator, which as a result improves its performance and make it reliable. Full article

Economic expedience of waste heat recovery systems (WHRS), especially for low temperature difference applications, is often questionable due to high capital investments and long pay-back periods. With a simple design, isobaric expansion (IE) machines could provide a viable pathway to utilizing otherwise unprofitable waste heat streams for power generation and particularly for pumping liquids and compression of gases. Different engine configurations are presented and discussed. A new method of modeling and calculation of the IE process and efficiency is used on IE cycles with various pure and mixed working fluids. Some interesting cases are presented. It is shown in this paper that the simplest non-regenerative IE engines are efficient at low temperature differences between a heat source and heat sink. The efficiency of the non-regenerative IE process with pure working fluid can be very high, approaching Carnot efficiency at low pressure and heat source/heat sink temperature differences. Regeneration can increase efficiency of the IE cycle to some extent. Application of mixed working fluids in combination with regeneration can significantly increase the range of high efficiencies to much larger temperature and pressure differences. Full article

A novel computational approach is developed to investigate the mixed convection, boundary layer flow over a nonlinear elastic (stretching or shrinking) surface. The viscous fluid is electrically conducting, incompressible, and propagating through a porous medium. The consequences of viscous dissipation, Joule heating, and heat sink/source of the volumetric rate of heat generation are also included in the energy balance equation. In order to formulate the mathematical modeling, a similarity analysis is performed. The numerical solution of nonlinear differential equations is accomplished through the use of a robust computational approach, which is identified as the Spectral Local Linearization Method (SLLM). The computational findings reported in this study show that, in addition to being simple to establish and numerically implement, the proposed method is very reliable in that it converges rapidly to achieve a specified goal and is more effective in resolving very complex models of nonlinear boundary value problems. In order to ensure the convergence of the proposed SLLM method, the Gauss–Seidel approach is used. The SLLM’s reliability and numerical stability can be optimized even more using Gauss–Seidel approach. The computational results for different emerging parameters are computed to show the behavior of velocity profile, skin friction coefficient, temperature profile, and Nusselt number. To evaluate the accuracy and the convergence of the obtained results, a comparison between the proposed approach and the bvp4c (built-in command in Matlab) method is presented. The Matlab software, which is used to generate machine time for executing the SLLM code, is also displayed in a table. Full article

This paper addresses a multiscale approach for heat recovery systems, used in two distinct applications. In both applications, a microscale approach is used (microchannel heat sinks and heat pipes) for macroscale applications (cooling of a photovoltaic—PV cell), and the thermal energy of exhaust gases of an internal combustion engine is used for thermoelectric generators with variable conductance heat pipes. Several experimental techniques are combined such as visualization, thermography with high spatial and temporal resolution, and the characterization of the flow hydrodynamics, such as the friction losses. The analysis performed evidences the relevance of looking at the physics of the observed phenomena to optimize the heat sink geometry. For instance, the results based on the dissipated heat flux and the convective heat transfer coefficients obtained in the tests of the microchannel-based heat sinks for cooling applications in PV cells show an improvement in the dissipated power at the expense of controlled pumping power, for the best performing geometries. Simple geometries based on these results were then used as inputs in a genetic algorithm to produce the optimized geometries. In both applications, the analysis performed evidences the potential of using two-phase flows. However, instabilities at the microscale must be accurately addressed to take advantage of liquid phase change. In this context, the use of enhanced interfaces may significantly contribute to the resolution of the instability issues as they are able to control bubble dynamics. Such an approach is also addressed here. Full article