Effect Of Heat Transfer Coefficient Research Articles

Comparison of heat transfer performance on closed pulsating heat pipe for Fe3O4 and ɤFe2O3 for achieving an empirical correlation

This paper describes the effect of heat transfer coefficient in an oscillating heat pipe for Fe3O4/water and ɤ (gamma) Fe2O3/kerosene. Experimental studies were performed to investigate the thermal performance of three oscillating heat pipes operating with heating power input in a range of 0–140W. The tested OHPs are all made from copper tubes with inner diameters (IDs) of 2, 2.5 and 3mm with different number of turns. Two working fluids, Fe3O4/water and ɤ (gamma) Fe2O3/kerosene, were used by filling ratios of 50%, by volume. Experimental results show that thermal performance of the OHPs depends on the conjugation effects of working fluid, inner diameter, heating power input and magnetic field. The 2.5mm ID CLOHPs had better thermal performance when charged with Fe3O4/water as compared with ɤFe2O3/kerosene. Finally, an empirical correlation based on 600 sets of available experimental data was proposed to predict the thermal performance of vertical CLOHPs for Fe3O4/water and ɤ (gamma) Fe2O3/kerosene.

Axisymmetric modeling of long pulsed laser heating with convective boundary conditions using analytical solutions

In this paper, we present an axisymmetric modeling of long pulsed laser heating with convective boundary conditions using analytical solutions. The two-dimensional axisymmetric physical model of the temperature rise problem for long pulse laser heating material with convective boundary conditions is established, and analytical solutions of the governing heat transfer equation are obtained by using the integral transformation method. It is found that the results obtained from the analytical solutions agree well with the existing finite element method. Temperatures of Silicon bulk for different convective boundary conditions are modeled, and effects of heat transfer coefficient and ambient temperature on the temperature distributions of Silicon are analyzed.

Effects of Heat Transfer Coefficients on Quenching Residual Stresses in 7055 Aluminum Alloy

The quenching process can produce great residual stresses in 7055 aluminum alloy plates. The main factor that affects the quenching residual stresses is the heat transfer coefficient in the quenching process. In this paper, the heat transfer coefficients of spray quenching under different spray water flows were measured by using the inverse method, and the heat transfer coefficients of immersion quenching under different water temperatures were measured by the iterative method. The heat transfer coefficient increases as the spray water flow increases while decreases as the water temperature increases. The basic differences of water temperatures/spray water flows/quenching methods are the different heat transfer coefficients. According to the heat transfer coefficients results of immersion and spray quenching, an orthogonal test was carried out to study the effects of heat transfer coefficients in different temperature regions on the quenching residual stresses. The heat transfer coefficients in the range of 100oC ~200oC have a great influence on the quenching residual stresses, especially for the heat transfer coefficient near 150oC.

A theoretical analysis of the impact of atmospheric parameters on the spectral, electrical and thermal performance of a concentrating III–V triple-junction solar cell

The spectral sensitivity of a concentrating triple-junction (3J) solar cell has been investigated. The atmospheric parameters such as the air mass (AM), aerosol optical depth (AOD) and precipitable water (PW) change the distribution of the solar spectrum in a way that the spectral, electrical and thermal performance of a 3J solar cell is affected. In this paper, the influence of the spectral changes on the performance of each subcell and whole cell has been analysed. It has been shown that increasing the AM and AOD have a negative impact on the spectral and electrical performance of 3J solar cells while increasing the PW has a positive effect, although, to a lesser degree. A three-dimensional finite element analysis model is used to quantify the effect of each atmospheric parameter on the thermal performance for a range of heat transfer coefficients from the back-plate to the ambient air and also ambient temperature. It is shown that a heat transfer coefficient greater than 1300W/(m2K) is required to keep the solar cell under 100°C at all times. In order to get a more realistic assessment and also to investigate the effect of heat transfer coefficient on the annual energy yield, the methodology is applied for four US locations using data from a typical meteorological year (TMY3).

Effect of heat transfer coefficient of steam turbine rotor on thermal stress field under off-design condition

The precise calculation of temperature and thermal stress field of steam turbine rotor under off-design conditions is of paramount significance for safe and economic operation, in which an accurate calculation of heat transfer (HT) coefficient plays a decisive role. HT coefficient changes dramatically along with working conditions. First, a finite element analysis of rotor model, applied with ordinary rotor materials, has been conducted to investigate the temperature and thermal stress difference along with the change of HT coefficient from 20 W/(m2∙°C) to 20000 W/(m2∙°C). Next, the differentiation between existing empirical formulas has been analyzed from the aspect of physical significance of nondimension parameters. Finally, a verifying case of the cold startup of a 1000MW unit has been proceeded. The result shows that the accuracy of coefficient calculation when steam parameters are low has a greater influence on that of rotor temperature and thermal stress, which means a precise empirical HT coefficient formula, like the Sarkar formula is strongly recommended. When steam parameters are high and HT coefficient is larger than 104 W/(m2∙°C), there will be barely any influence on the calculation of thermal stress. This research plays a constructive role in the calculation and analysis of thermal stress.

Performance analysis of cylindrical metal hydride beds with various heat exchange options

A 3D numerical heat-and-mass transfer model was used for the comparison of H2 uptake performances of powdered cylindrical MH beds comprising MmNi4.6Al0.4 hydrogen storage material. The considered options of heat exchange between the MH and a heat transfer fluid included internal cooling using straight (I) or helically coiled (II) tubing, as well as external cooling of the MH bed without (III) and with (IV) transversal fins. The dynamic performances of these layouts were compared based on the numerical simulation. The effect of heat transfer coefficient was also analysed.

Theoretical analysis of screened heat pipes for medium and high temperature solar applications

A mathematical model is applied to study the cylindrical heat pipes (HPs) behaviour when it is exposed to higher heat input at the evaporator for solar collector applications. The steady state analytical model includes two-dimensional heat conduction in the wall, the liquid flow in the wick and vapour hydrodynamics, and can be used to evaluate the working limits and to optimize the HP. The results of the analytical model are compared with numerical and experimental results available in literature, with good agreement. The effects of heat transfer coefficient, power input, evaporator length, pipe diameter, wick thickness and effective pore radius on the vapour temperature, maximum pressure drop and maximum heat transfer capability (HTC) of the HP are studied. The analysis shows that wick thickness plays an important role in the enhancement of HTC. Results show that it is possible to improve HTC of a HP by selecting the appropriate wick thickness, effective pore radius, and evaporator length. The parametric investigations are aimed to determine working limits and thermal performance of HP for medium temperature solar collector application.

Experimental and numerical heat transfer studies on quenching of Inconel 600 probe

The effect of heat transfer coefficient and quench start temperature on cooling behaviour of Inconel 600 quench probe was assessed by numerical experiments. A quantitative model that relates the mean cooling rate and quench start temperature of the probe with the boundary heat transfer coefficient was proposed. Computed aided cooling curve analysis was carried out by heating Inconel 600 probe to temperatures varying from 100 to 850 °C followed by quenching in water. The results of quenching experiments and the data available in the literature were used to validate the proposed model. A good agreement between the measured and estimated value was observed. The results showed that the film and transition boiling of cooling stages were significantly influenced by quench start temperature of the material while nucleate boiling and convective cooling stages were strongly dependent on the boundary heat transfer coefficient.

Thermal analysis of the melting process in a nuclear fuel rod

Melting of nuclear fuel rods is a crucial issue that must be addressed when conducting simulation analyses of severe accidents in nuclear reactors. The present work aims to develop a mathematical model and carry out numerical simulation of one-dimensional heat conduction with phase change in a nuclear fuel rod including two regions: uranium dioxide fuel pellet and zirconium alloy cladding. The temperature-dependent specific heats, thermal conductivities and densities are considered in the present investigation. The enthalpy formulation of the melting process in the nuclear rod during full reactor power operation is solved by using a finite-difference method. The thermal analysis is divided into four different phases: (1) transient phase before the cladding melting, (2) cladding melting phase, (3) transient phase before the fuel melting and (4) fuel melting phase. Then, the effect of heat transfer coefficient between coolant and fuel rod on the temperature histories of the fuel pellet and the cladding is investigated. Finally, the melting process of a nuclear rod with decay reactor power after shutdown is also simulated.

Thermal stresses in a multilayered thin film thermoelectric structure

This work investigates thermally induced stresses and deformations in a multilayered thin film thermoelectric (TE) structure for assessing thermomechanical reliability of the TE devices. The multilayered structure consists of a n-type and p-type thermocouple separated by an insulating layer, and an additional supporting membrane layer. The one-dimensional thermoelectricity is used to compute the temperature distribution with heat transfer by radiation and convection at the film surfaces being considered. The thermoelasticity theory of laminated composites is employed to determine the stresses and deformations in the thin film structure. It is found that the thermal stress calculated using the present laminate model has a significantly higher magnitude than that predicted by the strength of materials model. Effects of heat transfer coefficient at the film surfaces are also examined.

Feasibility and optimization of aerogel glazing system for building energy efficiency in different climates

Aerogel glazing system has become a promising energy-efficient window glass owing to its extremely low thermal conductivity and high visual transmittance. Two newly developed aerogel glazing systems (viz. #4 and #5) and three common glazing systems (viz. clear glass #1 and insulating glass #2 and #3) are investigated on the component heating and cooling load of a model building to evaluate their feasibility in various climates in China by using eQuest code. The building energy efficiency of 20, 11 and 9% could be obtained in Harbin, Beijing and Shanghai, respectively, as clear glass #1 were replaced with aerogel glazing system #5. The results demonstrated that the aerogel glazing system was extremely feasible in Severe Cold Region and significantly feasible in Cold and Hot-summer Cold-winter Regions. Effects of heat transfer coefficient (U) and shading coefficient (SC) of the glazing system on building energy loads were evaluated to further optimize glazing systems. The results demonstrated that the total heating load in colder climate reduced by 73% as the U of glass was decreased from 5.5 to 0.5 W/(m2 K). The total cooling load in warmer climate reduced by 16% as the SC was increased from 0.916 to 0.423. The findings could be of great potential in the selection and optimization of the glazing system in different climate zones aiming at building energy efficiency.

Heat transfer study for ITER blanket shield block cooling design

Two simplified models were developed for the cooling design of ITER shield block. Moreover, a new model, circular cylinder centered in a square solid, was also adopted to estimate the temperature, where the effects of heat transfer coefficient and volumetric heat rate were separated and studied individually. After that, the impact of dimension on the heat transfer in the new model was studied by a series of numerical analyses. At the last part, a numerical steady-state thermal analysis of a typical full shield block (SB) was performed to verify these models. Comparisons of the results from numerical analysis with these models show that the difference is acceptable in the practical application. The methods can be used not only for the cooling design, but also to know about the heat transfer in the SB.

Numerical Simulation Study for Background Plasma in the GAMMA 10 End-Mirror Cell

We have started a simulation study of background plasma in the GAMMA 10 west end-cell using a fluid-code in order to understand divertor simulation experiments. In this fluid-code, the configuration of magnetic field lines in the west end-cell of GAMMA 10 is adopted with a mesh structure and basic physical processes such as reflection coefficient and recombination are considered. An initial computation result reveals that in the case of the plasma density range: ne ~ 1019 m-3, ion and electron temperature could not be reduced significantly to the suitable temperature for achieving the detachment. The effect of heat transfer coefficient on the target plasma is also discussed.

45. Effect of heat transfer coefficient on cooling rates during oocyte vitrification in liquid and slush nitrogen

Advances in cryopreservation of gametes indicate that slow cooling protocols will soon be replaced by fast cooling methods. Vitrification, a process in which liquid water is converted into a glass-like amorphous solid without any ice formation, normally requires plunging directly into liquid nitrogen in order to achieve high cooling rates. This leads to strong nitrogen vaporization around the sample surface forming a vapor film that acts as a heat insulation layer (Leidenfrost effect) resulting in limited heat transfer coefficient (h) between the surface of the sample and liquid nitrogen. Recently, slush nitrogen (SN2), a mixture of solid and liquid nitrogen obtained by applying negative pressure (average −207 °C) has been introduced in an attempt to minimize this effect. The objective of this study was to conduct a comparison of numerically calculated cooling rates of two small-volume devices plunged in liquid nitrogen versus slush nitrogen, and to analyze the individual effects of a lower temperature (−207 °C) and higher h values. A survey of literature h values for film boiling of small metal objects with different geometries plunged in liquid nitrogen, revealed a range between 125 and 1000 W/(m2 K). These h values were applied to a numerical simulation of cooling rates of two oocyte vitrification devices (open-pulled straw and Cryotop®), plunged in conditions representative of vitrification processes in liquid and slush nitrogen. The heat conduction equation with convective boundary condition was numerically solved using the finite element method to simulate the cooling process (Comsol Multiphysics® software). The cooling rate (°C/min) was defined as the time needed to reduce initial core temperature (warmest point of the system) of the liquid from 20 °C to −150 °C while avoiding ice formation (vitrification); therefore the thermophysical properties of the vitrifying solution and the plastic device were considered independent of temperature. Numerical simulations were carried out with different h values likely to represent the stagnant and slush nitrogen conditions. The thermal properties for supercooled water at −23 °C were used; thermal conductivity (k) 0.50 W/m K, specific heat (Cp) 4218 J/kg K, and density (ρ) 983 kg/m3. For plastics (polyethylene) the following values at 23–25 °C were used: k = 0.22 W/m K, Cp = 1680 J/kg K, and ρ = 900 kg/m3. Predicted cooling rates for open-pulled straw and Cryotop® when cooled at −196 °C (liquid nitrogen) or −207 °C (slush nitrogen) indicated that lowering the cooling temperature produces only a maximum 10% increase in cooling rate confirming that the main benefit of plunging in slush over liquid nitrogen does not arise from their temperature difference. Numerical simulations also predicted that the fourfold increase in the cooling rate of vitrification devices when plunging in slush nitrogen would be explained by an increase in h values; this physical condition can be attributed to the fact that there is less or null film boiling when a sample is placed in slush nitrogen because it first melts the solid nitrogen before causing the liquid to boil and form a film. Funding sources: CONICET – Universidad Nacional de La Plata – Agencia de Promocion Cientifica y Tecnológica (ANPCYT) – Universidad Católica Argentina.

Evaluation of the convective heat transfer coefficient for minimum quantity lubrication (MQL)

PurposeThe purpose of this paper is to evaluate the cooling ability of minimum quantity lubrication (MQL) cutting fluid.Design/methodology/approachAn experimental system is devised to find the heat transfer coefficient of MQL under simulated reaming conditions. Cooling rate of the specimen is measured with an infrared camera. The effect of air pressure and oil volume on cooling rate is tested. Metal cutting tests are performed to evaluate the effect of heat transfer coefficient on workpiece temperature.FindingsConvective heat transfer coefficient for MQL increases with increasing air pressure. Oil volume has an indeterminate effect on the heat transfer coefficient; however, it is a dominant factor for controlling temperature during reaming.Practical implicationsThe results of the study can provide guidance to optimize the temperature controlling ability of MQL for production.Originality/valueThere is limited information available in literature regarding the heat transfer coefficient of metal working fluids, particularly for MQL. In particular, experiments designed to investigate the effect of air pressure and oil volume on the heat transfer coefficient of the mist have not been previously documented. This information may be used to improve the overall cooling ability of MQL mist, thus increasing its effectiveness at controlling tool wear and maintaining part quality. The other major contribution of this work is to separate the role of the cooling and lubrication for controlling temperature while reaming aluminum. Prior to this study, there has been relatively little research performed for the reaming metal cutting operation, and still less for reaming with MQL. The nature of how metal working fluids control temperature is not fully understood, and this work provides insight as to whether cooling or lubrication plays the dominant role for reaming.

Advanced Study of Non-Uniform Cell Voltage Distribution for a PEMFC Stack

In this paper, a fully coupled non-isothermal, electrochemical, and transport 3D model for a 10-cell PEMFC stack with coolant channels is constructed and implemented to examine and compare the influence factors to the stack performance. The first case to be considered is under different thermal operation conditions, including thermostatic, adiabatic, and heat exchange operation. The corresponding results show that a better uniformity and the largest stack output power density can be obtained under heat exchange operation. The other case is to compare the effects of heat transfer coefficients for different materials (ranging from 5 W/(m2·K) to 50 W/(m2·K)) on the spatial non-homogeneity of stack voltage and output power density. Numerical results indicate that the degree of the non-uniformity of individual cell voltage can be minimized, and the output power density can be elevated to a certain degree when the heat transfer coefficient is set as 25 W/(m2·K). In addition, an attempt is carried out to investigate the changes of some important variables due to the tolerance stacking or performance degradation, where we assume some cells’ contact resistance increases. We observe that a large jump of cell voltage and temperature occurs, which can be used as a detection signal for stack safety operation.

The Thermo-Economics Analysis of the Optimum Thickness of Double-Layer Insulation for Air Conditioning Duct

Problem statement: Engineering economics played an important role in engineering design works. The net saving value was the easiest criterion to indicate the optimum design. However, the information about the net saving analysis of double-layer insulation of air conditioning duct was insufficient. Thus, it was important to study this case. Approach: The optimum thickness analysis of air conditioning duct’s insulation, which composes of the layer of rubber and fiber glass insulator, was conducted by means of thermo-economics method. In addition, the effects of heat transfer coefficient at inside and outside of duct on the optimum thickness of these insulators were also studied. The research was done by considering the insulation’s optimum thickness of circular galvanized steel duct. The duct diameter of 0.5 m with rubber insulator (k = 0.035 W m-1 K-1) and fiber glass insulator (k = 0.045 W m-1 K-1) was selected to show the study results. In order to study the change in optimum thickness when convective heat transfer coefficients were varied, the inside and outside duct convective heat transfer coefficient of 6, 10, 14, 18 and 22 W m-2 Km in diameter and the inside and outside convective heat transfer coefficient of 6 and 22 W were selected for calculation of optimum thickness. Results: The results showed that the variation of inside and outside duct’s heat transfer coefficient did not affect on the insulator’s optimum thickness. In the case of a circular duct of 0.5 m in diameter and the inside and outside convective heat transfer coefficient of 6 and 22 W m-2 K-1 respectively, the optimum thickness was 0.0032 and 0.125 m for rubber and fiber glass insulator respectively. The net saving was 34,173.00 Baht per meter of duct. Conclusion: Finally, when the consideration in terms of insulator cost was done, the thermo-economics analysis of optimum thickness of double-layer insulation was recommended when the cost of main insulator was higher than that of auxiliary insulator otherwise the thermo-economics analysis of optimum thickness of single-layer insulation was sufficient. In addition, the variation of inside and outside duct convective heat transfer coefficient did not affect the optimum thickness. But net saving increased when inside and outside duct convective heat transfer coefficient increased.

The evaluation of reliability for zirconium diboride composite with additives of graphite

Statistical distribution of strength was investigated with different load speeds for ultra high temperature ceramic. It shows that the strengths under different load speeds follow Weibull probability well. Inertial stress was obtained for the ceramic under different load speeds. Finally, an application was implemented for the ceramic suffering quenching test. Dynamic thermal stress was obtained during quenching test, which was much higher than quasi-static one. The effect of heat transfer coefficients on failure probability range was evaluated for the ceramic during quenching test.

The Combined Effects of Heat Transfer Coefficient and Thermal Diffusivity on Heating Rates of Solid Materials - A Simple Analysis

It is often perceived that an object with a higher thermal diffusivity will heat faster in comparison to that with a lower thermal diffusivity. This is however not always the case, as previously numerically proved by Palazoglu (2006). In this short communication, an analytical explanation of this is provided. Three generic cases are discussed which reinforce the observations made by Palazoglu (2006).

C07 振動流中の熱伝達率に関する数値解析(パルス管冷凍機,熱音響機器及び関連要素と応用システム(3))

Numerical simulation of heat and fluid flow has been performed to clarify the effect of heat transfer coefficient on temperature change in oscillatory flows. Transient one-dimensional equations of continuity, momentum and energy are solved utilizing a TVD scheme. A basic pulse tube refrigerator is selected as the physical model. In this paper, we investigate the behavior of temperature changes during a cycle under various conditions of heat transfer coefficient. As a result, it was found that there are differences in temperature change among various conditions.