Unequal Heat Research Articles

Effective heat adaptation must engage with unequal heat risk

Effective heat adaptation must engage with unequal heat risk

Spatial Patterned Interfacial Solar Evaporators toward Recovering Heat Loss.

The novel freshwater production technology, such as interface solar-steam conversion (ISSC) technology, has advanced so rapidly recently, where its energy capture and conversion process was localized at the air-water interface so as to achieve high efficiency of energy utilization and transformation. However, when enlarging the evaporation surface and application scale, the inevitably increased heat loss and reduced conversion efficiency put it in a dilemma: should we exploit innovative steamer constructs for practical applications. In order to effectively mitigate heat loss from the evaporator to the surrounding environment, a series of spatial pattern evaporators (SPEs) are specifically designed in this article. By recovering the energy of radiation and convection heat loss, SPEs achieved low heat loss in an open evaporator through unequal height auxiliary heat exchange platforms. In an open environment, it achieves a maximum evaporation rate of 1.68 kg m-2 h-1, with approximately 52.41% of the heat loss being reabsorbed. This sophisticated pattern design provides a promising guideline for optimizing thermal management strategies and promoting practically scalable applications.

A novel microchannel-twisted pinfin hybrid heat sink for hotspot mitigation

Thermal hotspots cause excessive localized temperature rise, leading to significant temperature gradients across the microprocessor, which is the primary reason for its inadequate performance and early failure. This study proposes microchannel-pinfin hybrid heat sinks incorporating twisted and non-twisted pinfins of various cross-sectional shapes (hexagonal, pentagonal, square, and triangular) to demonstrate energy-efficient hotspot mitigation in a microprocessor with highly non-uniform power distribution. Microchannels and pinfins are positioned at low- and high-heat-flux zones, respectively, to reduce the temperature variations utilizing their unequal heat transfer capabilities. Here, high- and low-heat-flux zones represent the microprocessor-core area (hotspot) and remaining chip area (background zone), characterized by heat fluxes of 300 W/cm2 and 50 W/cm2, respectively. The pinfin twisting angle varies from 0° to 360° at a step of 45°. Conjugate heat transfer analyses are conducted through numerical solutions of the continuity, Navier-Stokes, and energy equations. The performance of the proposed hybrid heat sinks is compared with the non-hybrid (NH) and hybrid circular pinfins (HCP) heat sinks at Re = 120–440. The hybrid heat sink featuring triangular pinfins twisted at 225° angle (HTP-225) exhibits a remarkable reduction of 48.2 % in the total thermal resistance (Rth) and 58.4 % in the temperature non-uniformity (δT,bs) as compared to the NH heat sinks at Re = 440. Compared to the HCP design, the HTP-225 design shows 26.9 % and 35.8 % lower Rth and δT,bs, respectively. Moreover, the HTP-225 heat sink outperforms the NH heat sink with 46.0 % and 57.0 % lower Rth and δT,bs, respectively, at an equal pumping power of 18.6 mW. Furthermore, the HTP-225 heat sink demonstrates a 96.4 % and 38.5 % higher critical hotspot heat flux dissipating capacity than the NH and HCP heat sinks, respectively, making it a viable and effective solution for alleviating hotspots in high-power-density devices.

Study on heat transfer characteristics of a dual-evaporator ultra-thin loop heat pipe for laptop cooling

The thermal management of laptops is challenged by the presence of multiple heat sources with non-uniform distribution and limited space for heat dissipation. This paper proposes the application of a 1 mm thickness dual-evaporator ultra-thin loop heat pipe (DE-UTLHP) for cooling laptops. The start-up performance and steady-state heat transfer performance of the dual-evaporator ultra-thin loop heat pipes were studied under equal or unequal heat loads. The results show that successful set-up can be achieved within the range of heat loads from 5 W–5 W to 20 W–20 W in the horizontal orientation. And it can achieve a maximum heat load of up to 22 W–22 W and a minimum thermal resistance of 0.69 °C/W. Furthermore, it was found that it exhibited better heat transfer performance in the vertical orientation compared to the horizontal orientation. The proposed dual-evaporator ultra-thin loop heat pipes can convert isolated and dispersed direct heat dissipation into centralized and integrated thermal management for future laptops.

Development of a multipurpose heat treatment apparatus favorable for the growth of solution-based large-size crystal

An inexpensive temperature-controlled apparatus has been developed with the intention to use it during the growth of any solution-based crystal. It consists of a metallic cell and an application-specific, stand-alone, and programmable electronic controller. That can purposely deliver unequal heating power independently to its three individual heating channels. The system was designed with the aid of an Arduino-Uno board. It was indigenously built within the laboratory with limited in-campus facilities and using locally available standard electronic modules and components. It may have diverse applications for controllable synthesis processes in scientific laboratories and its structural as well as electronic design can be modified easily according to any specific user requirement.

Inequality in behavioural heat adaptation: an empirical study with mobility data from the transport system in New York City, NY, USA

Heat exposure, which can negatively affect human health and wellbeing, is heterogeneous within US cities. However, little is known about who can avoid heat stress by adjusting their everyday behaviour. We aimed to analyse the effect of ambient temperature on mobility, specifically subway (ie, the underground railway system) use, in New York City, NY, USA, during 2014-19. For this empirical study, subway use across New York City was measured with turnstile data from the New York City Metropolitan Transportation Authority between Jan 1, 2014, and Dec 31, 2019. Passenger numbers were then aggregated to the zip code tabulation area (ZCTA) level. Daily observational climate data were obtained from the US National Weather Service between Jan 1, 2014, and Dec 31, 2019. Socioeconomic data at the ZCTA level originated from the American Community Survey 2019. We extracted data on population age, ethnicity, commuting, employment, median household income, rent, and health-insurance coverage. We used a fixed-effects panel-regression model to assess the influence of temperature on subway use in New York City, which was the main outcome of our study. We obtained data for 438 subway stations across New York City. After data cleaning and preprocessing, the final aggregated data sample consisted of 238 508 instances of subway use in 1955 days across 6 years for 122 ZCTAs, with 168 days missing in the raw data and 67 days removed as outliers. The results of the fixed-effects panel-regression analysis showed a strong, non-linear effect of daily maximum temperature on subway use. Subway use was highest at 11·5°C and substantially decreased for temperatures that were colder and warmer than that, with reductions reaching 6·5% (95% CI 2·5-10·5) for the coldest temperature (ie, -6·5°C) and 10·5% (6·0-14·0) for the hottest temperature (ie, 34·5°C). Reductions differed between weekdays and weekends, when residents generally had more freedom to adjust their behaviour. Neighbourhoods that were at a socioeconomic disadvantage experienced smaller or no reductions in mobility in heat; mobility increased in neighbourhoods with beach access. Our study showed that temperature had a strong, non-linear effect on subway use, but the magnitude of the effect on subway use was heterogeneous across areas of the city on warm days. Weaker avoidance of heat stress correlated with less privilege, indicating compounding health risks. Everyday behavioural adaptation to heat is therefore an effect pathway that contributes to unequal heat effects and should be explored in future research. The Volkswagen Foundation and the Werner Siemens Foundation.

Unequal urban heat burdens impede climate justice and equity goals

Unequal urban heat burdens impede climate justice and equity goals

Unequal residential heating burden caused by combined heat and power phase-out under climate goals

Unequal residential heating burden caused by combined heat and power phase-out under climate goals

Parametric Study on Axially Grooved Heat Pipe With Two Heat Sources and One Heat Sink With Multiple Branches

AbstractIn order to use single heat pipe for multiple heat sources in electronics and space applications, a multibranch heat pipe with two evaporators and a condenser is developed with 20 internal grooves as a wick structure. An experimental investigation is carried out in the present study for axially grooved multibranch heat pipe (AGMBHP) by considering various parameters to obtain the best possible operating condition. AGMBHP is investigated for different filling ratios (range, 75–200%) in a horizontal orientation for equal heat loads (0–240 W) and unequal heat loads (0–120 W) on evaporators. Heat pipe is tested for dynamic characteristics for an optimum filling ratio range of 125–150%. Further, condenser cooling flowrate analysis and different orientation study is carried out and compared for horizontal, gravity-assisted, antigravity, and compound positions to understand the performance of heat pipe. AGMBHP is capable to transmit 240 W heat load with a minimum resistance of 0.192 °C/W in horizontal orientation. The optimum condenser cooling water flowrate is found to be 5 ml/s under tested conditions. AGMBHP is capable of providing effective thermal conductivity as high as 31,824 W/m°C, which is quite suitable for electronics cooling. Orientation study revealed that horizontal orientation is comparatively better amongst all the other orientations for AGMBHP.

Simulation model of boiling heat transfer and flow maldistribution in parallel mini-channels heated unequally

• Focus on boiling flow in parallel mini-channels heated unequally. • Developed model predicts boiling heat transfer and flow maldistribution. • Unequal heating among channels causes deterioration of heat transfer rate. • Considering the wall temperature distribution across the channels is important. Heat exchangers using parallel refrigerant mini-channels have been developed recently to improve the performance of air conditioning systems. Due to the unequal heat loads among channels, the boiling flow is unevenly distributed across parallel mini-channels in evaporators. Because this phenomenon sometimes causes deterioration of the heat exchange performance and difficulty in performance prediction, thus, it is essential to understand the characteristics of boiling heat transfer under unequally heated conditions. In this study, a novel simulation model was developed to predict the heat transfer characteristics and flow maldistribution of refrigerants flowing in parallel mini-channels with different heat fluxes or wall temperatures among channels. The model predicted well the flow maldistribution and the local quality, heat transfer coefficient, and dryout region of the two parallel mini-channels heated unequally. The mean heat transfer rate of the channels decreased remarkably with the increasing difference in wall temperature between the channels. The validity of the model was confirmed by comparing the predicted heat transfer rate with the experimental results. Furthermore, for ten parallel mini-channels, the decrease in the channel-mean heat transfer rate was indicated by the model, and the rate of decline varied significantly depending on the wall temperature distribution across the channels.

Physical modeling and machine learning of ceiling maximum temperature rise induced by tandem heat sources with unequal heat release rates in a natural ventilation tunnel

The present work involved experimental studies of physical modeling as well as machine learning prediction of the characteristics of ceiling maximum temperature induced by adjacent heat sources with unequal fire heat release rates in a tunnel. The following are considered as variables in this study: separation distances between the adjacent heat sources, and unequal heat release rates between upstream fire sources (UFS) and downstream fire sources (DFS). It was found that, with the increasing separation distance between adjacent fires, the ceiling maximum temperature gradually decreases for the given total heat release rate (Q˙TFS). Meanwhile, with the increase in the ratio (Q˙UFS/Q˙TFS) of the upstream fire heat release rate (Q˙UFS) to the total heat release rate, the ceiling maximum temperature rise of the tunnel presents nonmonotonic changes: first a decrease and then an increase. From the dimensional analysis of adjacent tandem fires, a prediction model of ceiling maximum temperature rise induced by different separation distances and Q˙UFS/Q˙TFS ratios was proposed. The model has a good correlation with the experimental and previous research results. Furthermore, to perform the calculation quickly and facilitate rapid engineering design, we use machine learning methods to derive the maximum temperature rise of the tunnel ceiling. The fire heat release rate is predicted using a machine learning algorithm. The three parameters (Q˙UFS/Q˙TFS, ΔTmax, Hd) were paired with the maximum temperature rise of the ceiling and trained using the GRU neural network model. The results show that the machine learning algorithms has a good agreement with the verification experimental value, cmparisons with established physical models are also discussed. The research results can serve as a reference for an intelligent tunnel firefighting system.

Performance evaluation of an active pipe-embedded building envelope system to transfer solar heat gain from the south to the north external wall

The unequal heat gain in south and north orientation of a building causes higher energy consumption which should have been avoided. In this paper, an active pipe-embedded building envelope system is proposed to achieve heat redistribution between north and south rooms to reduce building heating load. The system is mainly composed of closed-loop pipes embedded in external walls which will absorb and transfer the solar heat gain from the south façade to the north. On cloudless days, the structure can effectively raise the surface temperature of the north wall, thus reducing the heat loss through the envelope. We present all the mathematical equations and a mathematical model validated against experimental data in the literature. To further check the feasibility of the system, the heat transfer of a pipe-embedded external wall and a conventional wall are compared, with the same configurations under typical weather conditions in five building climate zones of China. The results show that the heating load reduction during heating season for the room adopting this system relative to the one without it is 12.8% for hot summer and cold winter climate. For severe cold climate and cold climate, the heating loads in January are reduced by 4.6% and 8.7%, respectively. The system plays a minor role in reducing building energy consumption in the hot summer and warm winter zone. Besides, energy consumption of building in summer can also be reduced, although not as effectively as in winter. This study may guide a better design and control of low energy building in further research and practice.

Experimental investigation on the start-up performance of a loop heat pipe with three flat disk evaporators combined

To promote the heat transport applicability of the loop heat pipe(LHP) for high power and multi-heat sources, a multi-evaporator loop heat pipe(ME-LHP) system with three flat disk evaporators equipped with separate vapor lines was proposed and experimentally studied for the first time. The effects of the interaction between evaporators, the heat load distribution, and the gravity-assisted angle on the startup behavior of ME-LHP were investigated respectively. The results show that with the separated-vaporline structure, the adverse impact between evaporators was greatly reduced. The best performance appeared when all three evaporators were involved. At unequal heat load working conditions, the ME-LHP obtained the optimal startup performance when high heat load was applied on evaporators with a superior position in liquid separation. Besides, the outlet temperature of all evaporators tended to be consistent, even if multiple evaporators worked under unequal heat loads. The gravity-assisted angle affected the start-up process by changing the non-uniformity of liquid distribution. Compared with the gravity-assisted tilt θ = 10°, the highest performance was improved by 77 % for θ = 2°, and the peak load could reach 300 W. The present study may extend the application of the flat plane type of loop heat pipe for solving the multi-heat-sources dissipation problem.

Exploring surface urban heat island (SUHI) intensity and its implications based on urban 3D neighborhood metrics: An investigation of 57 Chinese cities

Excessive urban temperature exerts a substantially negative impact on urban sustainability. Three-dimensional (3D) landscapes have a great impact on urban thermal environments, while their heat conditions and driving factors still remain unclear. This study mapped urban 3D neighborhoods and their associated SUHI (surface urban heat island) intensities in summer daytime across 57 Chinese cities, and then explored their relationships, driving factors as well as implications. Nine categories of urban 3D neighborhoods existed in Chinese cities and the 3D neighborhood of High Density & Medium Rise (HDMR) contributed the largest share of urban areas. The distribution of 3D neighborhoods varied among cities due to their distinct natural and economic traits. The average SUHI intensity can amount to 4.27 °C across all Chinese 3D neighborhoods. High Density & Low Rise (HDLR) and HDMR presented higher SUHI intensities than other 3D neighborhoods in China. Urban green space (UGI) and building height (BH) had great influences on SUHI intensities. The relative contribution of UGI decreased with the increase of building density and building height, but BH presented the opposite trend. The interaction of urban 3D landscapes and function zones led to highly complicated urban thermal environments, with higher SUHI intensities in industrial zones. Besides, the SUHI intensities of 3D neighborhoods presented great diurnal and seasonal variations, with higher SUHI intensities in HDHR and HDMR at nighttime in winter and summer. What's more, urban residents may suffer unequal heat risk inside cities due to the deviations of SUHI intensities among different 3D neighborhoods. It could be a highly effective way to mitigate SUHI effects in cities by increasing urban greening and improving urban ventilation.

Effect of non-linear thermal radiation on MHD Casson fluid flow past a stretching surface with chemical reaction

The analysis is accomplished to discover the innovation of an incompressible magnetohydrodynamic Casson liquid flow driven by a stretchable sheet with convective boundary conditions. The variations of non-linear thermic heat, chemical response and unequal heat absorption/generation are reserved into account. The flow reckonings are mutated into a scheme of dimensionless non-linear ODEs with appropriate similarity variables. The elucidation of the problem is attained by captivating the help of the fourth-order RK-based shooting procedure. The bearings of influential parameters on velocity, thermal and solutal fields are revealed with the assistance of diagrams and also the physical measures skin friction, local solutal and thermic heat transport are revealed in a distinct chart. Escalating the non-linear thermal radiation parameter as well as the temperature ratio restriction escalates the fluid temperature. The rate of mass transport is extraordinary for the increase of chemical response parameter and Schmidt number.

Experimental investigation on the start-up performance of a novel flat loop heat pipe with dual evaporators

Thermal management for multiple heat sources has been becoming increasingly important, especially under high heat flux conditions. As a passive heat transfer device, loop heat pipes (LHP) with multiple evaporators does not consume extra energy, demonstrating a great potential in thermal management. This paper proposed a flat plate LHP with dual evaporators for the first time. Two flat plate evaporators were adopted, and both of them had their own porous wick, compensation chamber (CC) and vapor line. The condensed liquid converged, then moved along the main liquid line. The CC of two evaporators was joined by a tube, which can ensure the sharing of the returning liquid. To verify the performance of novel flat plate LHP, the start-up test was investigated under different heat load arrangements, involving only a single evaporate was applied heat load, two evaporators were applied with the equal or unequal heat load. Meanwhile, the operation stability was determined by testing the performance with the successionally variable heat load. This system has been shown to operate successfully at heating source temperature below 100 °C with heat load ranging from 10 W–10 W to 130 W–130 W (corresponding to the maximum heat flux of 13.52 W/cm2).

Experimental Investigations on Performance Analysis of a Wickless Thermosiphon Heat Pipe With Two Heat Sources and Multiple Branches

Abstract In an electronic circuit of laptops, supercomputers with multiple central processing units, spacecraft etc., it is required to arrange the cooling system for multiple heat loads in the smallest possible space in view of power-saving opportunities. In the present study, the experimental investigations are carried out on a wickless multi-branch heat pipe in gravity-assisted mode with two evaporators and one condenser on each of the individual branches. The start-up and dynamic characteristics were studied with different filling ratios (range, 40–70%), with equal heat loads (range, 0–200 W) and unequal heat loads (range, 0–100 W) on evaporators. The results are analyzed in terms of temperature variation in axial direction, thermal resistance, and heat transfer coefficient for a multi-branch thermosiphon heat pipe (MBTHP). It was found that the optimal filling ratio depends on the applied heat load under tested conditions. In a thermosiphon mode, the heat pipe was capable of transporting a maximum heat load of 210 W and maximum heat flux of 20.31 W/cm2 with the maximum evaporator temperature lower than 100 °C. The minimum wickless thermal resistance of heat pipe was found to be 0.21 °C/W at 50% filling ratio and 160 W and maximum total heat transfer coefficient was found as 6.33 k W/m2 °C.

Limit cycle analysis and maldistribution mitigation for multi-channel cooling system

Flow boiling in microchannel can be applied for large heat load dissipation due to its high heat transfer coefficient, compact structure, and low refrigerant flow. However, instabilities in microchannel cooling systems, like pressure drop oscillation and flow maldistribution, can deteriorate cooling performance, cause component damage, and result in unbalanced temperatures. In this study, we explore the effects of heat load on limit cycles and temperature distributions for microchannel cooling systems with single, two, and multiple channels. The objective is to analyze the pressure drop oscillation characteristics and understand the changing patterns of limit cycles at different operating conditions, based on which control strategies can be developed for flow instability suppression. Numerical results show that the limit cycles are identical for channels with equal heat loads and the same pressure drop characteristics. Thus, mass fluxes and temperatures are evenly distributed. For unequally heated channels, the one with the smallest heat load would have a limit cycle crossing both positive and negative regions of the pressure drop characteristic curve, while the limit cycles of the channels with larger heat loads would oscillate only at the positive slope region (the high vapor quality side). Based on the understanding of limit cycles for multiple channels, we investigated the temperature distributions and pressure drop oscillations under the individual and combined effects of thermal and mass flow connections among channels. Results show that the thermal and mass flow connections would reduce the temperature difference for channels experiencing unequal heat loads, and a strong enough connection would result in the same limit cycles and synchronized temperatures for all channels.

ANALYSIS OF TEMPERATURE REGIME OF LAND SURFACE FOR BYSTRYTSIA RIVER BASIN AND THE INFLUENCE OF TERRAIN ATTRIBUTES USING LANDSAT 8 DATA

Terrain morphology is a powerful factor influencing climate characteristics, which manifests itself at various scale levels. At the detailed scale, this effect is mainly due to the redistribution of solar radiation on surfaces and slopes of different slope and aspect, which causes their unequal heating, and local redistribution of air masses. Spatial distribution of land surface temperatures can be effectively studied using remote sensing data in far-infrared range, which can be recalculated into temperature values. The values of the land surface temperature in the Bystrytsia river basin were calculated with far-infrared channels of Landsat spatial images for three time slices: October 5, 2013, February 13, 2015, August 10, 2016. The statistical analysis has been carried out on the impact of terrain morphometric parameters and land surface type and characteristics on its temperature. To determine the relative influence of each specific factor, the method of hierarchical partitioning has been implemented with the hier.part package of R software environment. Significant seasonal differentiation of the influence of individual factors on temperature was revealed. During all the seasons of the year, absolute height appeared as the most significant factor among those analyzed. The influence of absolute height on temperature distribution was the strongest in autumn, somewhat weaker in winter and the weakest in summer. Artificial surfaces, dry grass and soil were heated more strongly despite their lower albedo due to smaller heat consumption by evaporation, whereas vegetation surface and wet soils were less heated. On the other hand, the influence of relative surface insolation, being differentiated by terrain elements appeared to be relatively weaker, which can be explained by the calculation method used (insolation has been calculated for the moment the images were taken, whereas surface heating takes some time). The influence of the level of surface moisture also appeared to be significant. Key words: land surface temperature, Landsat, Bystrytsia, morphometric parameters, hierarchical partitioning.

Heat transfer characteristics of MHD flow of Williamson nanofluid over an exponential permeable stretching curved surface with variable thermal conductivity

This study aims to investigate the flow of two-dimensional magnetohydrodynamics (MHD) Williamson nanofluid over the permeable exponential stretching curved surface with variable thermal conductivity and activation energy. The Navier Stokes Equation along with Williamson fluid model is used to govern the partial differential equations (PDEs) and to transfer these PDEs into ordinary differential equations (ODEs), appropriate similarity transformations are utilized. The obtained ODEs are solved numerically. The effect of the physical parameters that are Williamson fluid parameter, curvature parameter, permeability parameter, temperature factors (Biot, Brinkman, thermal conductivity, temperature ratio, temperature difference, radiation, unequal heat source/sink), suction/injection parameter, magnetic parameter, nanofluid parameters (Brownian motion, thermophoresis), activation energy factors (reaction rate, fitted rate constant), Schmidt number, on velocity, pressure, temperature, and concentration profiles are shown through graphs.