Experimental study of convective heat transfer during cooling with low air velocity in a stack of objects

During cooling with low air velocity (u0.2 m·s⁻¹) of a stack of foodstuffs (a few centimeters dimension), the radiation and conduction between products can be of the same order of magnitude as convection. A method was developed to quantify these various transfer modes. The experiment was carried out using an in-line spherical arrangement; however, the same methodology can be applied to other product shapes. The results confirm that the heat transfers by radiation and conduction cannot be neglected. In addition, the convective heat transfer coefficient varies not only with air velocity but also with the product position in the stack.     

Numerical study on convective flow in a three-dimensional enclosure with hot solid body and discrete cooling

The aim of the present numerical study is to analyze the effect of cooler location and aspect ratio and position of the hot solid body inside the enclosure on three-dimensional natural convection flow in a cubical enclosure. The cooler and heater positions and aspect ratio of the heater in a cavity are examined under different combinations of partially cooling vertical sidewalls and, hot solid body in the cubical cavity. That is, (i) different cooler locations with a fixed size of the hot solid body, and, (ii) centrally located hot solid body with different aspect ratio. The three-dimensional convective flow and thermal arrangements in the enclosure are analyzed using the distribution of streamlines, isosurfaces, and Nusselt number. It is found that the cooler location and aspect ratio of hot solid body play a key role on convective cooling and energy transport inside the enclosure. The unit aspect ratio of hot solid body provides higher energy transport inside the enclosure for all cooler positions.     

Experimental study on the effects of acoustic streaming of high frequency ultrasonic waves on convective heat transfer: Effects of transducer position and wave interference

In the present study, an experimental study was carried out in order to find the effect of high frequency ultrasound wave transducer position on heat transfer rate. For this purpose, a thin platinum wire, which was used as a heater and sensor, was submerged in deionized water. The high frequency ultrasound transducers were installed at various positions including; two on lateral wall and three at the bottom of a cylindrical container. The interference effects of multiple transducers were investigated and it was found that a single transducer installed at the lateral wall works more efficiently than other layouts from heat transfer enhancement point of view.     

Investigation of the ground thermal potential in tunisia focused towards heating and cooling applications

In this paper, an experiment has been conducted in order to record the ground temperature at different depths during 2006 in a suburb of Sfax (Tunisia) which represents an example of the South-Mediterranean climate. The temperature of the soil has also been calculated using a thermal model taking into account properties of the soil and meteorological conditions. Experimental results are compared with theoretical predictions. In order to estimate the influence of the soil properties on the ground temperature, different soil thermal conductivities are tested. A simplified model of an earth pipe system is developed. The cooling and heating capabilities produced by such a system are evaluated. This model is validated against an other published experimental model.     

Cooling effect of thermally significant blood vessels in perfused tumor tissue during thermal therapy

This purpose of this article was focused on the cooling effects of thermally significant blood vessels on the extent of thermal lesion during heating treatments. The thermal modeling here based on the Pennes bio-heat transfer equation, describing the heat transfer of perfused tumor tissue, and the energy transport equation governing the heat convection and diffusion of the blood flow. The explicit finite difference method was used to solve the transient equation for the temperature field of a perfused tumor tissue encompassing a blood vessel in an axis-symmetric configuration during thermal therapy. As a result of simulation, the short-duration high-intensity heating is more effective on covering the treated tumor inside with a blood vessel 200 μm in diameter. For a blood vessel inside tumor tissue with a diameter larger than 2 mm, it is observed that neither longer heating duration nor higher heating power density is sufficient for complete necrosis of tumor.     

Influence of acoustic streaming on thermo-diffusion in a binary mixture under microgravity

An analytical and numerical study of the influence of acoustic streaming on species separation of a binary mixture under microgravity is presented. A rectangular cell filled with binary fluid is submitted to an ultrasonic propagating wave along a portion of one of its small walls while the opposite wall is perfectly absorbing. A temperature gradient is applied between the two other walls. The unicellular flow induced by the Eckart streaming may lead to significant species separation. In a first part, the hypothesis of parallel flow is used to determine the analytical solution which describes the unicellular flow and the separation is calculated analytically based on the acoustic streaming parameter, A, the acoustic beam width, ?, and the Schmidt number, Sc. Theses analytical results are corroborated by direct numerical simulations. In a second part, a linear stability analysis of the unicellular flow is performed. The eigenvalue problem resulting from the temporal stability analysis is solved by the Galerkin method, a spectral Tau–Chebychev method and by a finite element method. The thresholds for the stationary and oscillatory instability depend on the normalized acoustic beam width.     

Numerical study on thermal behavior and a liquid cooling strategy for lithium-ion battery

Investigation on the thermal behavior of the lithium-ion battery which includes the temperature response, heat contribution and generation, is of vital importance for their performance and safety. In this study, an electrochemical-thermal cycling model is presented for a 4 Ah 21700 type cylindrical single cell and 3× 3 battery pack and the model is validated by experiment on a single cell. Thermal behavior on a single cell is first analyzed, the results show that the heat generated in the charge is smaller than the discharge, and the polarization heat contributes the most to total heat, especially under higher rate. It can also be concluded from the battery pack that the temperature of the cell inside the battery pack is significantly greater than the external battery, while the temperature difference exists the opposite regular due to the worst heat dissipation of the central cell. Finally, after taking the enhanced liquid cooling strategy, the maximum temperature is 320.6 K that is reduced by 9.38%, and the maximum temperature difference is 4.9 K which is reduced by 69.6% at 2C, meeting the requirements of battery thermal management system.     

Further study on the thermal characteristic of a buried waxy crude oil pipeline during its cooling process after a shutdown

Since the temperature drop of waxy crude oil after a shutdown determines whether the pipeline is able to restart successfully or not, it is necessary to calculate the temperature drop and clarify the characteristic of the thermal process of waxy crude oil pipeline after the shutdown. However, the relevant techniques proposed in the previous researches for this calculation are not accurate enough, due to the complex phase change, non-Newtonian behavior of the fluid, and the transition of different heat transfer mechanisms involved within the physical problem. Therefore, in a companion piece to this paper, a general and accurate mathematical model was proposed for the phase-change heat transfer of waxy crude oil. In this paper, the mathematical model of the waxy crude oil pipeline system after its shutdown is established, based on the phase-change heat transfer model proposed in the companion piece, and the numerical procedure is established for the calculation of the model. With the proposed techniques, the thermal process of the shutdown of waxy crude oil is investigated in detail, and the temperature drop characteristic is clarified on the level of heat transfer mechanism. The research will provide theoretical support for the establishment of shutdown scheme and thermal preservation method for waxy crude pipeline.     

Influence of convective cooling on the temperature in a frictionally heated strip and foundation

The analytical solution of a boundary-value heat conduction problem of friction for the tribosystem consisting of a semi-infinite foundation and a plane-parallel strip sliding over its surface is obtained. The evolution of temperature and its distribution in depth from a contact surface for materials of frictional couple, such as aluminum-steel, is studied.     

Influence of the convective cooling and the thermal resistance on the temperature of the pad/disc tribosystem

The analytical solution of a boundary–value problem of heat conductivity for a tribosystem, consisting of the plane–parallel strip (pad) sliding with a velocity linearly decreasing in time, on a surface of a semi-infinity foundation (disc) is obtained. It is assumed, that the thermal contact of bodies is imperfect and the convective heat exchange with the surrounding occurs on an upper surface of the strip. For materials of frictional pair, i.e. the ceramic–metal strip and the cast-iron foundation, the influence of the coefficients (of both heat exchange and heat transfer through a surface of contact) on the distribution of temperature has been investigated.     

装载机热管理仿真与试验研究

通过一维/三维耦合仿真方法进行装载机热管理仿真计算,建立装载机整车三维模型,利用Fluent商业软件计算发动机舱内的空气流动。计算结果表明:散热器进风不均匀,散热器上部与下部形成热风回流,整机左右两侧进风口的进风不畅和风扇周围形成多处涡流;利用三维仿真得到的散热器进口风量,在一维仿真中反推机舱阻力系数,利用KULI商业软件,计算发动机出水温度、液压油散热器进口温度、传动油散热器进口温度;通过仿真数据与试验数据的对比,验证了一维/三维耦合仿真的准确性。     

Mixed convective flow and thermal stratification in hot water storage tanks during discharging mode

An analysis of transient, two dimensional, mixed convection and thermal stratification in cylindrical hot water storage tanks is presented. The governing equations together with inflow and outflow boundary conditions are written for laminar mixed convection flow using a finite volume based computational code in the dynamic discharging mode based on Boussinesq approximations and conjugate heat transfer. The equations are solved numerically and the results are obtained for aspect ratios of the tanks ranging from 1 to 4 in the Richardson number range of 10⁵ to 10⁸ using a finite volume based computational code. The dynamic discharging mode is considered using a conjugate heat transfer model. The transient temperature profiles in the bulk fluid reveal reduced mixing at higher Richardson numbers during discharging process. The system performance in the dynamic mode of operation is defined by a Mix Number and discharging efficiency parameter. Mixing at the bottom of the tank due to inflow of low temperature water from the load is found to have significant influence on the storage efficiency. The discharging efficiency decreases with Fourier number due to increased thermal degradation with time.     

Numerical study of mixed convective cooling in a square cavity ventilated and partially heated from the below utilizing nanofluid

A numerical investigation of mixed convection flows through a copper–water nanofluid in a square cavity with inlet and outlet ports has been executed. The natural convection effect is attained by heating from the constant flux heat source which is symmetrical located at the bottom wall and cooling from the injected flow. The governing equations have been solved using the finite volume approach, using SIMPLE algorithm on the collocated arrangement. The study has been carried out for the Reynolds number in the range 50 ≤ Re ≤ 1000, with Richardson numbers 0 ≤ Ri ≤ 10 and for solid volume fraction 0 ≤ ? ≤ 0.05. The thermal conductivity and effective viscosity of nanofluid have been calculated by Patel and Brinkman models, respectively. Results are presented in the form of streamlines, isotherms, average Nusselt number and average bulk temperature. In addition, the effects of solid volume fraction of nanofluids on the hydrodynamic and thermal characteristics have been investigated and discussed. The results indicate that increase in solid concentration leads to increase in the average Nusselt number at the heat source surface and decrease in the average bulk temperature.     

Energy efficiency and indoor thermal perception: a comparative study between radiant panel and portable convective heaters

This study investigates experimentally the thermal perception of indoor environment for evaluating the ability of radiant panel heaters to produce thermal comfort for space occupants as well as the energy consumption in comparison with conventional portable natural convective heaters. The thermal perception results show that, compared with conventional convection heater, a radiantly heated office room maintains a lower ambient air temperature while providing equal levels of thermal perception on the thermal dummy head as the convective heater and saves up to 39.1% of the energy consumption per day. However, for human subjects’ vote experiments, the results show that for an environmentally controlled test room at outdoor environment temperatures of 0°C and 5°C, using two radiant panel heaters with a total capacity of 580 W leads to a better comfort sensation than the conventional portable natural convective heater with a 670 W capacity, with an energy saving of about 13.4%. In addition, for an outdoor environment temperature of 10°C, using one radiant panel heater with a capacity of 290 W leads to a better comfort sensation than the conventional convection heater with a 670 W capacity, with an energy saving of about 56.7%. From the analytical results, it is found that distributing the radiant panel heater inside the office room, one on the wall facing the window and the other on the wall close to the window, provides the best operative temperature distribution within the room.     

A numerical study on thermal analysis and cooling flow fields effect on PEMFC performance

Thermal management and water management are two important interconnected topics in the design and increase the efficiency of PEM fuel cells. Suitable cooling flow field design with proper performance is an important factor in increasing the lifetime of PEM fuel cell, because non-uniformity of temperature reduces the stability and durability of PEM fuel cell. Different cooling strategies are considered for removing of heat generation by PEM fuel cell, because the fuel cell temperature remains in a tolerable range and homogeneous as possible. In the first step, determine the value and location of heat sources in fuel cell, is important and appropriate cooling strategy can be applied. In this study, a PEM fuel cell with serpentine gas flow field is simulated with six different cooling flow fields simultaneously, e.g. conventional serpentine (Model 1), typical of MPSFFs (model 2), typical of serpentine (Model 3), parallel-serpentine (Model 4), conventional spiral (Model 5) and conventional parallel (Model 6). This simulation showes a correct perception of temperature distribution in PEMFC. The results indicate that Model 5 has a good temperature and performance based on the minimum and maximum temperature gradient, Index of uniform temperature (IUT), however has a more pressure drop. For second choice when the pressure drop is important, the Model 3 has a better performance than other models. Also, thermal analysis in these cases shows that ohmic, entropic and reaction heating included 20%, 35%, 45% of the total heat generation by PEMFC, approximately.     

冷却流量对涂层叶片冷却性能影响的数值研究

采用气热耦合数值方法研究了冷却流量对热障涂层气冷涡轮叶片冷却性能的影响,并对结果进行了对比分析。研究结果表明：热障涂层叶片的综合冷却效率随冷却流量的增加而增大,但增幅则逐渐下降。在吸力面上,附加热障涂层的效果更好。基准工况下,附加热障涂层,叶片表面温度可降低72.6 K,综合冷却效率增幅最大可达6.5%。在尾缘区域,热障涂层会阻碍热量从金属叶片表面向低温的流体传递,导致叶片表面性能下降,因此,只有配合高效的内冷技术,才能达到理想的冷却效果。     

Theoretical model on the heat and mass transfer in silica gel packed beds during the regeneration assisted by high-intensity ultrasound

In the present work, a theoretical model is reported on the heat and mass transfer in silica gel packed bed during the regeneration process by using hot air combined with high-intensity ultrasound. The model consists of two parts: one is about the sound propagation in porous media; the other is about the fundamental heat and mass transfer process in the silica gel (particle) packed bed. The theoretical model is then validated by experiments in terms of the exit air temperature and humidity (kg/(kg dryair)) under different conditions of regeneration. The experimental error due to the measuring instruments is estimated to be within 0.4% and 3.5%, respectively, for the exit air temperature and humidity. The comparison between theoretical and experimental data shows that the mean relative errors (MREs) of the calculated exit air temperature and humidity compared with the experimental ones are mostly within 2.0%, which manifests the model developed in this study has a favorite agreement with the experiments. The theoretical model will help us conduct a further parametric analysis on the regeneration process in the presence of an ultrasonic field, and have a better understanding of the mechanism of enhancement of silica gel regeneration brought by the high-intensity ultrasound.     

Numerical analysis of convective flow and thermal stratification in a cryogenic storage tank

In this study, the liquid–vapor mixture model was used for a numerical study of natural convective flow in a cryogenic tank with a capacity of 4.9?m³ under various conditions of heat flux and filling level to understand the early stages of convective flow phenomena and the consequent thermal stratification of cryogenic liquid. Two cryogens—liquefied natural gas (LNG) and liquefied nitrogen (LN₂)—were compared to observe their effects. LN₂ exhibited faster vaporization owing to its lower heat of vaporization. It was observed that higher heat flux and lower filling level led to faster vaporization and relatively vigorous heat transfer, showing early thermal stratification.     

经皮胆管穿刺引流与经内镜鼻胆管引流胆管冷却保护技术在肝癌微波消融中的应用对比

【摘要】 目的 比较经皮肝胆管穿刺引流术（PTCD）与经内镜鼻胆管引流术（ENBD）置管灌注冷却液在近肝门部肝癌微波消融（MWA）中防止胆管热损伤的作用。方法 收集上海东方肝胆外科医院2013年9月至2016年9月因近肝门部肝癌行经皮肝穿刺MWA的23例患者临床资料,其中12例经PTCD置管,11例经ENBD置管于肿瘤旁胆管,向置管内快速持续灌注４℃ 0.9%NaCl溶液,再在Ｂ超引导下行MWA治疗。比较两组胆管热损伤、冷却技术相关并发症及肝癌复发情况。结果 平均随访20个月（3～36个月）,23例患者均无手术相关死亡。两组各有1例术后形成胆汁瘤,PTCD组无明显冷却技术相关并发症,而ENBD组有4例（36.4％,P=0.037）:1例（9.1%）乳头切开后出血、3例（27.3%）急性胰腺炎。两组原位复发率、肝内复发率及病死率均无明显差异（P=1.00, 0.77及0.61）。结论 近肝门部肝癌根治性MWA中,经PTCD与ENBD置管灌注冷却液是防止胆管热损伤可行、有效的方法,前者技术相关性并发症更少。