bluuebunny/crossref_metadata_2025 huggingface dataset:
Extraordinary Thermal Conductivity of Graphene: Prospects of Thermal Management Applications
Authors: Ghosh Suchismita , Nika Denis L. , Pokatilov Evgenni P. , Calizo Irene , Balandin Alexander A.
Published: March 2011

We have recently discovered experimentally that suspended graphene, which is an individual sheet of sp2-hybridized carbon bound in two dimensions (2D), reveal an extremely high thermal conductivity. The measurements were performed using a non-contact optical technique developed by us on the basis of Raman spectroscopy. A large number of graphene flakes were suspended across trenches in Si wafers and attached to heat sinks. The flakes were heated by the focused laser light in the middle of the suspended portion of graphene. The amount of laser power dissipated in graphene and corresponding local temperature rise were determined from the integrated intensity and spectral position of graphene’s Raman G mode. The position of the G peak as a function of the sample temperature was measured independently allowing the use of micro-Raman spectrometer as a “thermometer”. The experimental thermal conductivity values were in the range of ∼ 3000–5300 W/mK near room temperature (RT) and depended on the graphene flake sizes. The thermal conductivity of graphene is the highest among all materials known to date. In this review work we will describe the details of our measurement procedure and explain theoretically why the 2D thermal conductivity of graphene is higher than that of bulk graphite provided that the size of graphene flakes is sufficiently large. Our theory, which includes the phonon-mode dependent Gruneisen parameter and phonon scattering on edges and defects, gives results, which are in excellent agreement with the experiment. Superior thermal properties of graphene are beneficial for the proposed graphene electronic devices, and may pave the way for graphene’s thermal management applications.
Steady State Modeling of LHP and Analysis
Authors: Chen Yan , Cheng Lin , Xin Gongming , Luan Tao
Published: March 2011

The loop heat pipe (LHP) was invented in Russia in the early 1980’s. It is a two-phase heat transfer device that utilizes the evaporation and condensation of a working fluid to transfer heat, and the capillary force developed in fine porous wicks to circulate the fluid. The temperature of LHP evaporator as functions of the heat load, sink temperature, ambient temperature is an important parameter which can reflect the performance of an LHP. Many factors can affect the LHP operating temperature and which can be divided into two parts: external and internal. The external factors including heat sink temperature, ambient temperature, fluid forces, the position between heat source and heat sink and the heat exchange between LHP and ambient. The internal factors related to the design and structure of the LHP, for example, the charging amount of the working fluid and the distribution status of the liquid phase during the LHP operating. Based on Sinda/Fluint software an ammonia-stainless steel steady state model of loop heat pipe was established, the impacts on the LHP operating temperature induced by alterable heat loads under 3 operating cases (the different position between evaporator and condenser, the changing of ambient temperature and the changing of heat sink temperature) were analyzed and conclusions were made. Changing the position between evaporator and condenser has a significant influence on the LHP operating temperature. Anti-gravity operation will reduce the performance of the LHP, this phenomenon is obviously in low heat load range. Further more, increasing of fluid pressure drop in the loop will induce decreasing of the LHP performance. The temperature difference between ambient and heat sink will influence the transition heat load (from variable conductance mode to fixed conductance mode), the bigger the temperature difference the higher the transition heat load.
Compact Modelling of Forced Convection in Pin/Plate-Fin Heat Sinks Subjected to Impinging Jet Heating
Authors: Feng Shangsheng , Kim Tongbeum , Lu Tianjian
Published: March 2011

Built upon the porous medium approach, a compact model is presented for forced convection in pin and plate fin heat sinks subjected to non-uniform heating. The modified Darcy’s law and the two-equation model are adopted separately to describe fluid flow and heat transfer in the porous model. To take account of heat spreading in the plate-fins and its absence in the pin-fins, the concept of anisotropic effective thermal conductivity is employed in the energy equation for the solid phase. To validate the model, experiments are conducted for both pin- and plate-fin heat sinks and the measured local temperature distribution on the heat sink substrate is compared with that predicted. Experimental results reveal that, due to additional heat spreading in the plate-fins, the substrate temperature of the plate-fin heat sink has a more uniform distribution than that of the pin-fin heat sink. Obtained good agreement between model predictions and experimental measurements suggests that the present model is suitable for predicting the spreading effects in the connected solid phase of porous media (e.g., plate-fins) under non-uniform heating boundary conditions.
Performance of Heat Driven Type Water Cooler Using Metal Hydride
Authors: Bae Sangchul , Haruna Yusuke , Nohmi Naoko , Ishikawa Keisuke , Katsuta Masafumi
Published: March 2011

In this study, we aim at developing the heat driven type water cooler using metal hydride (abbr., MH) alloy. Heat driven type MH water cooler is one of the chemical heat pumps, and the endothermic reaction on the cooling part MH (to put it simply, MH2) is used for the cooling. Because MH is too expensive (200∼300 $ per 1kg) and has an unfavorable activation characteristic, this cooler has not been used generally yet. In order to increase the system performance, we use a new TiFe alloy, which has been developed by co-researcher, to the heat source part MH (to put it simply, MH1). Moreover, to improve the cooling load per MH mass, we mix the brush type carbon fiber, 2 mass% into MH beds. By this method, the cooling load per MH mass is been increased to 0.078 kW/kg (MH2).
Surface Functionalization Mechanisms of Enhancing Heat Transfer at Solid-Liquid Interfaces
Authors: Hu Ming , Goicochea Javier V. , Michel Bruno , Poulikakos Dimos
Published: March 2011

Two mechanisms that increase heat dissipation at solid-liquid interfaces are investigated from the atomistic point of view using nonequilibrium molecular dynamics (NEMD) simulation. The mechanisms include surface functionalization, where −OH terminated headgroups and self-assembled monolayers (SAMs) with different chain lengths are used to recondition and modify the hydrophilicity of silica surface, and vibrational matching between crystalline silica and liquid water, where three-dimensional quartz nanopillars are grown at the interface in the direction of the heat flux with different lengths to rectify the vibrational frequencies of quartz surface atoms. The heat dissipation is measured in terms of the interfacial thermal conductance at the solid-liquid interface, whereas the thermal conductance is obtained by imposing a one-dimensional heat flux across the simulation domain. The heat dissipation is enhanced by a factor of 2 to 3 for both fully hydroxylated and pillar modified surfaces. The SAMs enhance the overall thermal conductance between silica and water further (20% higher thermal conductance compared to the fully hydroxylated silica surface). Moreover, the modification of the vibrational states at the silica surface provides a tunable path to enhance the heat dissipation, which can also be easily applied to other fluids.
Flow Boiling Heat Transfer of R123/R134a Mixture in a Micro-Channel
Authors: In Sehwan , Jeong Sangkwon
Published: March 2011

This paper describes the flow boiling heat transfer of R123/R134a mixture in a single round micro-channel with 0.19 mm ID. The flow boiling heat transfer coefficients were measured with the variation of mixture composition (R123 mole fraction: 0.502, 0.746) at various experimental conditions: mass velocities (314, 392, 470 kg/m2-s), heat fluxes (10, 15, 20 kW/m2) and vapor qualities (0.2–0.85). The heat transfer characteristics of R123/R134a mixture are similar to those of pure R123 observed in the previous flow boiling experiment. The similarity of heat transfer characteristics denotes that the heat transfer is governed by evaporation of thin liquid film around the elongated bubbles like the case of pure R123. The heat transfer coefficients of R123/R134a mixture are compared with those of equivalent pure refrigerant by the correlation developed from pure R123 experimental results. The large reduction of heat transfer coefficients compared with pure refrigerant is found in micro-channels flow boiling by the mass transfer effect of mixed refrigerant. In addition, macro-channel correlations for mixed refrigerant do not make accurate prediction about the reduction of heat transfer coefficients.
Numerical Simulation of Heat Transfer and Fluid Dynamics in Supersonic Chemically Reacting Flows
Authors: Molchanov Alexander M. , Arsentyeva Anna A.
Published: March 2011

An implicit fully coupled numerical method for modeling of chemically reacting flows is presented. Favre averaged Navier-Stokes equations of multi-component gas mixture with nonequilibrium chemical reactions using Arrhenius chemistry are applied. A special method of splitting convective fluxes is introduced. This method allows for using spatially second-order approximation in the main flow region and of first-order approximation in regions with discontinuities. To consider the effects of high-speed compressibility on turbulence the author suggests a correction for the model, which is linearly dependent on Mach turbulent number. For the validation of the code the described numerical procedures are applied to a series of flow and heat and mass transfer problems. These include supersonic combustion of hydrogen in a vitiated air, chemically reacting flow through fluid rocket nozzle, afterburning of fluid and solid rocket plumes, fluid dynamics and convective heat transfer in convergent-divergent nozzle. Comparison of the simulation with available experimental data showed a good agreement for the above problems.
Vacuum Operation of a Thermosyphon Reboiler
Authors: Heggs Peter John , Alane Abdelmadjid
Published: March 2011

The research facility at the University of Manchester in the Morton Laboratory is a full scale replica of an industrial sized natural circulation thermosyphon reboiler, which comprises 50 tubes of 3 m length and 25.4 mm OD. The facility is operated under vacuum. Water is used as the process fluid and condensing steam is the heating source. Experimental datasets were obtained for the reboiler and have been presented in the form of profile plots of feed rate, fluid recirculation, recirculation ratio and vapour quality. The data elucidate the effect of pressure [0.1 to 1.0 bar] and heat duties [78 to 930 kW] on the performance of the reboiler. Three distinct modes of operation have been observed. Mode one is defined as a flow-induced instability or geysering (low heat duty) and exists below a definite transitional point that is independent of process pressure. Mode two is a region of stable operation that occurs above the threshold of the flow-induced instability, while mode three, which is defined as the heat-induced instability (density-wave instability), is pressure dependent obtained at high duties and is characterised by violent oscillations. These instability thresholds represent the lower and upper limits of operation of the reboiler. The region of stable operation is enveloped between the two limits and is very dependent on process pressure as it progressively becomes smaller as the vacuum becomes lower. These studies led to unique experimental observations, which revealed the existence of intermittent reversed flow in the entire loop. The use of throttling in the heat-induced unstable region to return to stable operation tends to be over a narrow range, outside of which the sole way to regain stability is to lower the heat load or increase the process pressure. In the region of flow-induced instability, throttling the fluid at the inlet is useless and actually makes the situation worse. These instabilities are alleviated by increasing the heat load.
Evaluation of a Coaxial Borehole Heat Exchanger Prototype
Authors: Acun˜a Jose´ , Mogensen Palne , Palm Bjo¨rn
Published: March 2011

Different borehole heat exchanger designs have been discussed for many years. However, the U-pipe design has dominated the market, and the introduction of new designs has been practically lacking. The interest for innovation within this field is rapidly increasing and other designs are being introduced on the market. This paper presents a general state of the art summary of the borehole heat exchanger research in the last years. A first study of a prototype coaxial borehole heat exchanger consisting of one central pipe and five external channels is also presented. The particular geometry of the heat exchanger is analyzed thermally in 2-D with a FEM software. An experimental evaluation consisting of two in situ thermal response tests and measurements of the pressure drop at different flow rates is also presented. The latter tests are carried out at two different flow directions with an extra temperature measurement point at the borehole bottom that shows the different heat flow distribution along the heat exchanger for the two flow cases. The borehole thermal resistance of the coaxial design is calculated both based on experimental data and theoretically.
Numerical Heat Transfer Studies of Flows Over Concave Surfaces
Authors: Souza Leandro F. , Liu Joseph T. C.
Published: March 2011

One of the main objectives of the researchers is to obtain a heat transfer rate enhancement for flows with a minimal increasing in the drag coefficient. The boundary layer over concave surfaces is unstable to centrifugal forces, giving rise to Go¨rtler Vortices. These Vortices create two regions in the spanwise direction, the upwash and downwash regions. The downwash region is responsible to compress the boundary layer in the wall direction, increasing the heat transfer rate. The upwash region does the opposite. In the nonlinear development of the Go¨rtler Vortices it can be observed that the upwash region becomes narrow, and the average heat transfer rate is higher than that for a Blasius boundary layer. In the present work Spatial Direct Numerical Simulations are carried out in order to quantify the heat transfer coefficient of flows over concave surfaces. Three different wavelength values were used to verify the influence of this parameter in the heat transfer rate. The results were obtained for gaseous and liquid media, using Prandtl numbers equal 0.72 and 7.07 for that, respectively. The results show that the heat transfer rate can achieve values close to turbulent boundary layers without increasing too much the drag coefficient.
Thermal Behavior of Phase Change Material During Charging Inside a Finned Cylindrical Latent Heat Energy Storage System: Effects of the Arrangement and Number of Fins
Authors: Groulx Dominic , Ogoh Wilson
Published: March 2011

One way of storing thermal energy is through the use of latent heat energy storage systems. One such system, composed of a cylindrical container filled with paraffin wax, through which a copper pipe carrying hot water is inserted, is presented in this paper. It is shown that the physical processes encountered in the flow of water, the heat transfer by conduction and convection, and the phase change behavior of the phase change material can be modeled numerically using the finite element method. Only charging (melting) is treated in this paper. The appearance and the behavior of the melting front can be simulated by modifying the specific heat of the PCM to account for the increased amount of energy, in the form of latent heat of fusion, needed to melt the PCM over its melting temperature range. The effects of adding fins to the system are also studied, as well as the effects of the water inlet velocity.
Experimental Investigation of Louver Cooling Scheme on Gas Turbine Vane Pressure Side
Authors: Elnady T. , Saleh W. , Hassan I. , Kadem L. , Lucas T.
Published: March 2011

An experimental investigation has been performed to measure the cooling performance of the louver scheme using a two-dimensional cascade simulating the scaled vane of a high-pressure gas turbine. Two rows of an axially oriented louver scheme are distributed in a stagger arrangement over the pressure side. The effect of hole location on the cooling performance is investigated for each row individually, then the row interaction is investigated for both rows. The temperature distribution on the vane is mapped using a transient Thermochromic Liquid Crystal (TLC) technique to obtain the local distributions of the heat transfer coefficient and film cooling effectiveness. The performance of the louver scheme for each case is compared with that of two similar rows with a standard cylindrical exit at 0.9 density ratio. The exit Reynolds number based on the true chord is 1.5E5 and exit Mach number is 0.23. The local distributions of the effectiveness and the heat transfer coefficient are presented at four different blowing ratios ranging from 1 to 2. The louver scheme shows a superior cooling effectiveness than that of the cylindrical holes at all blowing ratios in terms of protection and lateral coverage. The row location highly affects the cooling performance for both the louver and cylindrical scheme due to the local pressure change and the variation of the surface curvature.
Dimensionless Groups and Simulations for Heat and Mass Transfer in Adsorbed Natural Gas Storage
Authors: Sphaier L. A.
Published: March 2011

Adsorbed natural gas (ANG) has been emerging as an attractive alternative to compressed natural gas and liquefied natural gas, on various circumstances. In spite of the advantages associated with ANG over other storage modes, there are some issues that need be properly addressed in order to ensure a viable employment of such alternative. One major problem is that the thermal effects associated with the sorption phenomena tend to diminish the storage capacity, thereby resulting in poorer performance. Hence, in order to design commercially viable storage vessels, the heat and mass transfer mechanisms that occur in these devices must be carefully understood and controlled. In this context, this work presents different mathematical models for simulating ANG charge and discharge processes. For the sake of improving the understanding of mass and energy transport within ANG vessels, dimensionless groups associated with this problem are also discussed. By using the proposed formulations, simulation results were numerically calculated and the process of charging ANG vessels was analyzed for different combinations of the presented dimensionless groups. Pressure, temperature and stored mass histories are compared, showing that the unwanted heating effects can be minimized for certain values of the dimensionless groups. Finally, a comparison between the results obtained using the two different formulations suggests that there may be cases in which the simpler model could be employed.
Heat and Fluid Flow in Expanding Concentric Annular Flow-Passage With Rotating Inner-Wall
Authors: Murata Akira , Iwamoto Kaoru
Published: March 2011

An annular flow passage with a rotating inner wall is widely seen in many industrial apparatuses. In this type of passages, the flow is disturbed by the multiple reasons: the centrifugal forces due to the streamline curvature in the through-flow and circumferential directions, the centrifugally driven flow in the through-flow direction depending on the passage inclination, and the adverse pressure gradient due to the flow-passage expansion resulting in the flow separation. In this study, heat and fluid flow in this type of annular flow passage was numerically investigated. The large eddy simulation with a Lagrangian dynamic subgrid-scale model was performed with a finite difference method. Two different flow passages (concave and convex types) were examined. Inlet through-flow Reynolds number was Rein = 1000 and the Taylor number was varied among Ta = 0, 1000, 2000, and 4000. When the rotation speed was gradually increased, the flow changed into spiral structure at first and then fragmented vortex structure in the downstream region. The flow structure changed depending on the passage type, the Taylor number, and the position in the through-flow and wall-normal directions. The Nusselt number showed a complicated profile, and near the outlet the Nusselt number on the outer wall became very high because of the low-temperature inflow compensating the centrifugally driven outflow near the rotating inner wall.
Experimental Study on a Flat Loop Heat Pipe Coupling the Compensation Chamber and the Condenser
Authors: Mo Dong-chuan , Zou Guan-sheng , Lu Shu-shen
Published: March 2011

Loop heat pipes are high efficient heat transfer loops/devices. Compared to the typical loop heat pipe with cylinder evaporator, loop heat pipe with flat evaporator (flat loop heat pipe, FLHP) can reduce the thermal resistance between the evaporator and the heat loads. In order to remove the heat leak from the evaporator to the compensation chamber to reduce the operation temperature, a new type of FLHP coupling the compensation chamber and the condenser has been developed. Experiments have been conducted to compare the heat transfer characteristics between the normal type and the new type of FLHP. Part of the heat lead from the evaporator to the compensation chamber can be removed in the new type of FLHP, so it gives better heat transfer performance than the normal one. Results show that, the temperatures in the loop of the new type of FLHP are much more stable than the normal one. The evaporator temperatures and the total thermal resistances of the new type are much lower than those of the normal type. For the normal type of FLHP, it may be failed to start up under low power, and usually the larger temperature oscillation will happen. With the power increasing, the frequency of the oscillation is increasing. When the applied power is large enough, the loop can keep running in the design way, and the large temperature oscillation will disappear.
Study on a Cascade Utilization of Liquid Natural Gas (LNG) Cold Energy for Power Generation
Authors: Lu Yuanwei , Yang Hongchang , Ma Chongfang
Published: March 2011

Liquefied natural gas (LNG) is known as a clean energy source which is widely used in electricity generation and daily living for its friendly environment performance. The global LNG trade has increased rapidly during recent years. Liquid natural gas delivered by sea-ships contains considerable cryogenic exergy which can be utilized for electricity production before its evaporation and introduction into the system of pipelines. Because the need to vaporize LNG offers a thermal sink at a very much lower temperature than seawater, using this thermal sink both power and gas can be produced. So it is possible to recover power from the vaporization of LNG. Based on the exergy analysis of normal Rankine power cycle and LNG gasification characteristics at a different pressure, this paper proposed a cascading power cycle with LNG cold energy recovery, in which a three-tier Rankine cycle system was established to recovery the liquid heat and latent heat of LNG, and the residual sensible heat of LNG vapor can be used to prepare the cold water of central air-conditioning in plant area. The simulating results showed that the cascade power system can improve the LNG cold energy recovery efficiency.
Analysis of Transport Processes and Chemical Reaction in Combustion Duct of Compact Methane Reformer
Authors: Yang Guogang , Wei Wei , Yuan Jinliang , Yue Danting , Lv Xinrong
Published: March 2011

A composite combustion duct in compact methane reformers consists of a gas flow channel, porous layer and solid plates. There are various transport processes appeared, such as gas flow in the channel, multi-component species convection/diffusion in the porous layer, and heat transfer. They are further coupled by methane catalytic combustion in the porous layer, which affects the reformer overall performance and reliability. By three dimensional CFD approach, the reacting gas flow and heat transfer processes were numerically studied. The reformer conditions such as mass balances associated with the chemical reaction and gas permeation to/from the porous layer are implemented in the calculation. The results reveal that the catalytic combustion reaction is confined in a thin porous catalyst area close to fuel gas flow duct. Transport processes of the fuel gas species and temperature distribution are significantly affected by the reactions.
Mass Nature of Heat and Its Applications V: Entransy, Entransy Dissipation and Heat Transfer Irreversibility
Authors: Liang Xin-Gang , Chen Qun
Published: March 2011

Heat transfer optimization is ubiquitous because improving heat transfer performance could increase the energy utilization or reduce the weight or size of heat transfer equipments. This article discusses the optimization in heat transfer using the new physical quantity, entransy, in recent years. Entransy describes the heat transfer ability. When heat is transferred from a high temperature to a low temperature and entransy dissipation is produced. Heat transfer is irreversible from the viewpoint of entransy. The entransy transfer efficiency can be defined using the concept of entransy. Definition of entransy, entransy flux, and entransy dissipation are given and the entransy balance equations are derived for conduction, convection and thermal radiation based on the energy equation. The minimum entransy dissipation principle for prescribed heat flux boundary conditions and a maximum entransy dissipation principle for prescribed temperature boundary conditions are investigated. These two principles are called entransy dissipation extreme (EDE) principle. An equivalent or average thermal resistance of a system can be defined based on the entransy dissipation and the EDE principle becomes the minimum thermal resistance principle. These principles can be used to optimize heat transport with constraints and some examples are presented. The relation of entransy with thermomass is discussed and comparison between EDE and entropy generation optimization is made. The essence of the entansy is the energy of thermomass.
Heat Transfer and Pressure Drop During Hydrocarbon Refrigerant Vaporisation Inside a Brazed Plate Heat Exchanger
Authors: Longo Giovanni A.
Published: March 2011

This paper presents the experimental heat transfer coefficients and pressure drop measured during HC-600a (Isobutane), HC-290 (Propane) and HC-1270 (Propylene) vaporisation inside a small brazed plate heat exchanger: the effects of heat flux, refrigerant mass flux, saturation temperature (pressure), outlet conditions and fluid properties are investigated. The experimental tests include 172 vaporisation runs carried out at three different saturation temperatures: 10, 15 and 20°C. The refrigerant mass flux ranges from 6.6 to 23.9 kg/m2s and the heat flux from 4.3 to 19.6 kW/m2. The heat transfer coefficients show great sensitivity to heat flux, outlet conditions and fluid properties and weak sensitivity to saturation temperature (pressure). The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow and therefore a quadratic dependence on refrigerant mass flux. HC-1270 shows heat transfer coefficients 6–12% higher than HC-290 and 35–50% higher than HC-600a and frictional pressure drops 5–10% lower than HC-290 and 2.5 time lower than HC-600a. The experimental heat transfer coefficients are compared with two well-known equations for nucleate boiling and a correlation for frictional pressure drop is proposed.
Design Optimization and Validation of Single-Phase Rectangular Micro Channels With Axial Non-Uniform Width
Authors: Van Oevelen Tijs , Baelmans Martine
Published: March 2011

A simple model for the simulation of the hydraulic behaviour and heat transfer of a single micro channel is formulated, assessed and optimized. The model is based on a thermally developing flow regime. The results of the optimization show that a reduction of 7.8% in thermal resistance can be achieved using non-uniform channels instead of uniform channels. Furthermore, the assumptions of the simple model are validated by comparing the results with a more accurate, numerical model. It is shown that the simple model is sufficiently accurate to simulate the hydraulic and thermal behaviour of micro channels as such. However, the effective improvement of channel width optimization cannot accurately be determined by the analytical models.
Influence of Favourable Pressure Gradient on the Heat Transfer in Boundary Layer
Authors: Sakhnov Alexey Yu. , Volchkov Eduard P. , Makarov Maxem S.
Published: March 2011

In the present paper, we analyze, both numerically and analytically, the influence a favourable pressure gradient has on the characteristics of dynamic boundary layer. It is shown that at a certain value of the pressure gradient asymptotic conditions can be reached in laminar boundary layer, with the skin-friction coefficient value being independent of the Reynolds number, like in the case of asymptotic suction of boundary layer through the porous wall. Some analogy between the two types of flow can be traced: in both cases, namely, at porous suction and in the boundary layer of an accelerated flow, a cross flow is generated, directed from the outer edge of the boundary layer toward the wall. For the conditions of the two types, an approximate analytical solution to the boundary-layer equations has been obtained. It is shown that in problems with first- and second-kind boundary conditions the favourable pressure gradient exerts an influence on the heat-transfer characteristics.
Experimental Investigation on the Plate Heat Pipe Heat Exchanger for Heat Recovery Characteristic in Room Ventilation
Authors: Yu Wen-Jing , Diao Yan-Hua , Zhao Yao-Hua
Published: March 2011

A plate HPHE (heat pipe heat exchanger) has been designed and constructed for heat recovery in this paper. The plate HPHE is constituted by plate heat pipes with microchannels. Many parameters affect the performance of the HPHE. In this paper, the effect of three parameters: inlet air temperature (27–40°C), working fluid (methanol, acetone, R113, R141b), and heat exchange area (single layer, double layer) were investigated experimentally. The overall experiments were carried out under the simulated summer condition, the vacuum of the HPHE was set at 1×10−3Pa, and the filling ratio was 1/3. The air volume rates were changed from 60, 90, 120 and 150 m3/h. The fresh air temperature varied from 27 to 40°C, whereas the return air temperature was kept at about 24°C. The results show that, at the same air volume, the effectiveness increases with increase of the inlet temperature of fresh air. The effectiveness of HPHEd with R141b as working fluid is superior to R113, acetone and methanol, and the maximum effectiveness reaches to 58%. The effectiveness of the HPHE of single layer is lower than that of double layer by about 6%–18% at the temperature from 29 to 40°C.
Flow-Rate and Reaction-Temperature Effects Upon Ethanol Steam Reforming With Cu/ZnO/Al2O3
Authors: Okuhigashi Yuki , Shinoki Toshio , Funaki Jiro , Hirata Katsuya
Published: March 2011

The authors reveal the dominant chemical reactions and the optimum conditions, supposing the design of ethanol steam-reforming reactors. Specifically, experiments are conducted for Cu/ZnO/Al2O3 catalyst, together with those for Ru/Al2O3 catalyst for reference. Using a household-use-scale reactor with well-controlled temperature distributions, the authors compare experimental results with chemical-equilibrium theories. As a result, the Cu/ZnO/Al2O3 catalyst shows rather high performance at low values of reaction temperature T. This suggests that the Cu/ZnO/Al2O3 catalyst promotes the ethanol-steam-reforming and water-gas-shift reactions, but does not promote the methanation reaction. Furthermore, the authors have researched the influence of liquid-hourly space velocity LHSV upon the ethanol conversion XC2H5OH in the range of LHSV from 0.05 to 1.40h−1, S/C = 3.0 and T = 420K, 470K and 520K, and the influence of LHSV upon concentrations such as CH2, CCO2, CCO and CCH4 in the range of LHSV from 0.05 to 1.20h−1, at S/C = 3.0 and T = 470K. In addition, the authors have proposed a new and simple method to recover the catalyst performance.
A Numerical Study of the Effect of a Horizontal Frame Member on the Laminar and Turbulent Natural Convective Heat Transfer From a Recessed Window to a Surrounding Room
Authors: Oosthuizen Patrick H. , Naylor D.
Published: March 2011

The vertical and horizontal frame members that often protrude from the inner surface of a window can, in some situations, have a significant effect on the convective heat transfer rate from the inner (room-side) surface of the window to the room. The purpose of the present numerical study was to determine, in a basic way, how the relative size of a single horizontal frame member mounted in the center of the window affects this convective heat transfer rate. A recessed window has been considered. The flow has been assumed to be steady and both laminar and turbulent flows have been considered. Fluid properties have been assumed constant except for the density change with temperature that gives rise to the buoyancy forces, this being dealt with using the Boussinesq approach. The governing equations have been solved using the FLUENT commercial cfd code. The k-epsilon turbulence model with standard wall functions and with buoyancy force effects fully accounted for has been used in the calculations. The solution has the following parameters: the Rayleigh number, the Prandtl number, the dimensionless horizontal distance between the inner window surface and the inner surface of the wall in which the window is mounted (the dimensionless recess depth), and the dimensionless width and depth of the frame member. Results have only been obtained for a Prandtl number of 0.74, which is effectively the value for air, and for single values of the dimensionless window recess depth and of the dimensionless frame height. The effects of the other dimensionless variables on the window Nusselt number have been numerically studied.
A Numerical Study of Laminar and Turbulent Mixed Convective Flow Over a Vertical Isothermal Plate
Authors: Oosthuizen Patrick H.
Published: March 2011

Mixed (or combined) convective flow is flow with heat transfer in which there is a forced flow but in which the buoyancy forces that arise due to temperature variations in the flow have a significant effect on the flow and therefore on the heat transfer rate. In such flows the buoyancy forces can also have a very significant influence on the conditions under which transition from laminar to turbulent flow occurs. In the present study this effect of the buoyancy forces on the conditions under which transition occurs have been studied for the particular case of flow in the vertically upward direction over a heated vertical flat plane surface that is maintained at a uniform temperature that is higher than the temperature of the undisturbed fluid flow, i.e., attention has been restricted to assisting (or aiding) mixed convective flow. The flow has been assumed to be steady and it has also been assumed that the fluid properties are constant except for the density change with temperature which gives rise to the buoyancy forces, this having been treated by using the Boussinesq approach. The solution has been obtained by numerically solving the governing equations subject to the boundary conditions using the commercial cfd solver, FLUENT. The k-epsilon turbulence model with full account being taken of the buoyancy forces has been used in obtaining the solutions. The mean heat transfer rate from the surface expressed in terms of the mean Nusselt number depends on the Reynolds number based on the free-stream forced velocity and the length of the heated surface, on the Rayleigh number based on the length of the heated surface and the overall surface to free-stream temperature difference, and on the Prandtl number. Results have only been obtained for a Prandtl number of 0.74. Solutions have been obtained for a series of increasing Rayleigh numbers between 105 and 1012 for a series of Reynolds numbers between approximately 1 and 107.
Comparison of Single and Two-Phase Models for the Study of Mixed Convection Flows With Nanofluids
Authors: Akbari Mahmood , Behzadmehr Amin , Galanis Nicolas
Published: March 2011

The single phase and three different two phase models (Volume of fluid, Mixture and Eulerian) are used to analyse laminar mixed convection flow of Al2O3-water nanofluids in a horizontal tube, in order to evaluate their prediction ability. The flow is considered steady and developing. The fluid’s physical properties are temperature dependent whereas those of the solid particles are constant. A uniform heat flux is applied at the fluid-solid interface. Two different Reynolds numbers and three different volume fractions have been considered. The governing three-dimensional partial differential equations are elliptical in all directions and coupled. Predicted convective heat transfer coefficients, velocity, and temperature profiles, as well as secondary flow’s velocity vectors and temperature contours are compared at different axial positions. To validate the comparisons and verify the accuracy of the results, the numerical predictions are compared with corresponding experimental data. There are essentially no differences between the predictions of the two-phase models; however their results are significantly different from those of the single-phase approach. Two-phase model results are closer to the experimental data, but they show an unrealistic increase in heat transfer for small changes of the particle volume fraction. Hydrodynamically, the two-phase and single-phase approaches perform almost the same but their thermal predictions are quite different.
Heat Transfer Enhancement of Phase Change Materials (PCMs) in Low and High Temperature Thermal Storage by Using Porous Materials
Authors: Zhao C. Y. , Zhou D. , Wu Z. G.
Published: March 2011

In this paper the solid/liquid phase change heat transfer in porous materials (metal foams and expanded graphite) at low and high temperatures is experimentally investigated, in an attempt to examine the feasibility of using metal foams to enhance the heat transfer capability of phase change materials for use with both the low and high temperature thermal energy storage systems. In this research, the organic commercial paraffin wax and inorganic hydrate calcium chloride hydrate salts were employed as the low-temperature materials, while the sodium nitrate is used as the high-temperature PCM in the experiment. The heat transfer characteristics of these PCMs embedded with open-cell metal foams were studied experimentally. The composites of paraffin and expanded graphite with different graphite mass ratios, namely, 3%, 6% and 9%, were also made and the heat transfer performances of these composites were tested and compared with metal foams. Overall metal foams can provide better heat transfer performance than expanded graphite due to their continuous inter-connected structures. But the porous materials can suppress the natural convection effect in liquid zone, particularly for the PCMs with low viscosities, thereby leading to the different heat transfer performance at different regimes (solid, solid/liquid and liquid regions). This implies that the porous materials don’t necessarily mean they can always enhance heat transfer in every regime.
Understanding the Hazards of Grouped Electrical Cables
Authors: McGrattan K. , Lock A. , Marsh N. , Nyden M. , Dreisbach J. , Stroup D.
Published: March 2011

CHRISTIFIRE (Cable Heat Release, Ignition, and Spread in Tray Installations during FIRE) is a U.S. Nuclear Regulatory Commission Office of Research program to quantify the mass and energy released from burning electrical cables. This type of quantitative information will be used to develop more realistic models of cable fires for use in fire probabilistic risk assessment (PRA) analyses. The experimental program has two main thrusts—bench-scale measurements of small samples of burning cables and full-scale measurements of the heat release and fire-spread rates of cables burning within typical ladder-type trays. The bench-scale measurements include micro-calorimetry of cable components, effluent characterization using absorption spectroscopy, and measurements of the heat release rate using a cone calorimeter. The full-scale measurements include the burning of a variety of cables within a typical tray under radiant panel heating, and full-scale, multiple tray fires. The outcome of the experiments is to be used by a variety of fire models, ranging from simple correlations to computational fluid dynamics.
Transient Evaporation of Liquid Water Films and Condensation of Humid Air at the Isothermal Walls of a Square Cavity
Authors: Sun Hua , Lauriat Guy , Nicolas Xavier
Published: March 2011

The aim of this paper is to present modelling and numerical simulations of heat and mass transfer by double-diffusive convection and surface condensation or evaporation in a two-dimensional square cavity subjected to a uniform, but time-dependent wall temperature. At initial state, the cavity is filled with quiescent humid air at uniform temperature and density. By decreasing the wall temperature, condensation occurs at the four wall surfaces. The walls are then heated and evaporation of the liquid water is considered. Since the mass of humid air and average pressure experience noticeable changes during the periodic process, a weakly compressible formulation has been used for the modelling of the thermosolutal flow. Local thermodynamic assumption at the liquid-vapor interface and thin liquid film approximation are introduced.
Effect of Rain on Evolution of Distribution of Soluble Gaseous Pollutants in the Atmosphere
Authors: Krasovitov Boris , Elperin Tov , Fominykh Andrew , Vikhansky Alexander
Published: March 2011

We suggest a model of rain scavenging of soluble gaseous pollutants in the atmosphere. It is shown that below-cloud gas scavenging is determined by non-stationary convective diffusion equation with the effective Peclet number. The obtained equation was analyzed numerically in the case of log-normal droplet size distribution. Calculations of scavenging coefficient and the rates of precipitation scavenging are performed for wet removal of ammonia (NH3) and sulfur dioxide (SO2) from the atmosphere. It is shown that scavenging coefficient is non-stationary and height-dependent. It is found also that the scavenging coefficient strongly depends on initial concentration distribution of soluble gaseous pollutants in the atmosphere. It is shown that in the case of linear distribution of the initial concentration of gaseous pollutants whereby the initial concentration of gaseous pollutants decreases with altitude, the scavenging coefficient increases with height in the beginning of rainfall. At the later stage of the rain scavenging coefficient decreases with height in the upper below-cloud layers of the atmosphere.
Analysis of Visible Radiative Properties of Vertically Aligned Multi-Walled Carbon Nanotubes
Authors: Bao Hua , Ruan Xiulin , Fisher Timothy S.
Published: March 2011

Finite-difference time-domain method is used to investigate the optical properties of vertical arrays of multi-walled carbon nanotubes. Individual carbon nanotubes are treated as solid circular cylinders with an effective dielectric tensor. Our results confirm that carbon nanotube arrays have extremely low reflectivity as observed in experiments. Compared with the commonly used Maxwell-Garnett theory, our calculations generally give larger reflectance and absorptance and smaller transmittance, which are attributed to the diffraction and scattering within the cylinder array structure. The effects of volume fraction, tube length, and incidence angle on radiative properties are investigated respectively. Low volume fraction and long tubes are more favorable to achieve low reflectance and high absorptance. The angular dependence study shows that there exists an optimum incidence angle at which the reflectance can be minimized, indicating that a small misalignment in carbon nanotube arrays can slightly enhance the absorptance. Our results also indicate that an even darker material could be achieved by using carbon nanotubes with good alignment on the top surface.
Experimental Investigation on Heat Transfer and Pressure Drop of Nanodiamond-Engine Oil Nanofluid in a Microfin Tube
Authors: Akhavan-Behabadi M. A. , Ghazvini M. , Rasouli E.
Published: March 2011

In this study, the effect of adding nanodiamond powder as an additive to engine oil on laminar flow heat transfer enhancement and pressure drop increasing is experimentally investigated. The plain and microfin tubes were used as the test sections and were heated by an electrical coil heater to produce constant heat fluxes. Thermal conductivity and heat capacity of nanofluids were measured for different volume fractions and temperatures. Convection heat transfer coefficients and Nusselt numbers of nanofluids were obtained for different nanoparticle concentrations as well as various Peclet and Reynolds numbers. Experimental results show the enhancement of heat transfer due to the nanoparticles presence. Furthermore, the effect of particle concentration on pressure drop was studied for different heat fluxes. Finally, the performance evaluation of both nanofluid and microfin tube from the point view of heat transfer enhancement and pressure drop increasing is done.
An Improved Generalized Model for Predicting Frost Growth on a Cold Flat Plate
Authors: Guo Q. C. , Wang W. , Xiao J. , Lu W. P.
Published: March 2011

A generalized quasi-steady and one-dimensional model for predicting the frost growth on flat plate was proposed based on the previous theoretical models. To improve the predicting ability of the current model, a modified semi-empirical correlation for calculating initial condition of frost density was presented experimentally. The experiments were conducted in a suction-type open-loop wind tunnel under a series of experimental conditions: air temperature −8°C to 19°C, humidity 42% to 80%, velocity 5m/s and the temperature of cold plate −16°C to −8°C. The numerical results of frost thickness, frost density, frost surface temperature and heat flux rate were compared to the experimental data. The simulation results were found agree with the experimental results in a maximum error of 10%. The presented model was further validated by comparing with the previous published experimental data in a wide range of frosting conditions. It was found that the presented model was a simple but universal one to predict the frost growth on cold flat plate.
Quantitative Evaluation of Transport Properties of SOFC Porous Anode and Their Effect on the Power Generation Performance
Authors: Kishimoto Masashi , Iwai Hiroshi , Saito Motohiro , Yoshida Hideo
Published: March 2011

The three-dimensional microstructure of a solid oxide fuel cell (SOFC) anode is directly observed using a focused ion beam and scanning electron microscope (FIB-SEM) technique. Microstructural parameters, which are closely related to transport phenomena in porous materials, are quantitatively evaluated by a random-walk-based diffusion simulation. Numerical simulation of the SOFC anode with the obtained microstructural parameters is also performed, and the result is in good agreement with the experimental counterparts. Combined with a sensitivity analysis for the SOFC performance, the relationships between the microstructural parameters and the power generation performance are discussed and guidelines for optimizing the anode microstructure are proposed.