Reduction In Heat Transfer Coefficient Research Articles

Modulation of near-field radiative heat transfer between graphene sheets by strain engineering

In this article, we study the near-field radiative heat transfer (NFRHT) between two graphene sheets under mechanical strain. The modulation of NFRHT due to the strain modulus and stretching direction is investigated under two types of strain configurations. For the first type, one graphene sheet is strained whereas the other one is unstrained. It is found that the spectra of NFRHT undergo a redshift and the magnitudes drop remarkably as the strain modulus increases. For the second type of configuration, two graphene sheets have the same strain modulus while the stretching direction could be arbitrary. It is found that the differences in stretching directions lead to the mismatch of anisotropic plasmonic modes. Under proper choices of stretching directions, a large modulation with the reduction of heat transfer coefficient over 60% is possible for strain modulus 0.2. Our findings may have promising applications in thermal management for micro/nano-electromechanical devices.

Thermal and Aerodynamic Researches of Staggered Bundles for Choice of an Effective Spacing of Round-Finned Tubes

The results of an experimental study of local modeling of convective heat transfer and aerodynamic resistance of staggered six-row bundles of bimetallic tubes with spiral knurled aluminum fins under transverse air flow in the range of its velocity alteration in a compressed bundle section of 1.9−11.0 m/s are presented. The velocity range covers the possible modes of operation of industrial air coolers (AVO). The fins with a diameter of approximately 57 mm are rolled on a steel supporting tube with an outer diameter of 25 mm. Tube finning ratio j = 19.26. Such tubes are widely used in the heat exchange sections of AVO of natural gas, in particular, at “Gribanovskii Engineering Plant” JSC (Russia). To measure the reduced heat transfer coefficients, an electric calorimeter had been developed by the authors with a power input of 600−1300 W. The temperature of the wall surface at the base of the fins did not exceed the range of 77–92 °C. The transverse tube spacing in bundles S1was 64.0 or 68.0 mm, while the longitudinal spacing S2 was 54.4 or 50.0 mm. The heat transfer of each transverse row of six-row bundles was measured, as well as the average heat transfer and aerodynamic drag, which are summarized by the similarity equation of a power type. The heat transfer rate of the last transverse row in the direction of air movement is 0–5 % lower than the heat transfer rate of the stabilized rows, and here new features of heat transfer variations in the insufficiently studied area of spacing changes S1 and S2 have been found. The thermal contact resistance (TCR) was measured in the range of the average temperature of the contact surfaces tк = (79–95) оС, and no dependence of the value of TCR on tк for the specified interval was found. The numerical average value of TCR was Rк = 2,13 × 10–4 m2×K/W, which is typical for reliable mechanical connection of the finned aluminum shell with the supporting steel tube made of carbon steel. The results of variant thermal and aerodynamic calculations with the use of the obtained data established the technical and economic feasibility of placing tubes at the vertices of an isosceles triangle with spacing S1 = 68–69 mm and S2 = 55 mm with failure to use the location of the tubes along an equilateral triangle with S1 = S2' = 64 mm (where S2' – is diagonal spacing). With Q = idem and other conditions being equal, the number of tubes on AVO decreases by 5.7 % with a decrease in power consumption to 4.0 %.

Oil fouling in double‐pipe heat exchanger under liquid‐liquid dispersion and the influence of copper oxide nanofluid

AbstractDispersions of oil in water are encountered in a variety of industrial processes leading to a reduction in the performance of the heat exchangers when thermally treating such two phase fluids. This reduction is mainly due to changes in the thermal and hydrodynamical behavior of the two phase fluid. In the present work, an experimental investigation was performed to study the effects of light oil fouling on the heat transfer coefficient in a double‐pipe heat exchanger under turbulent flow conditions. The effects of different operating conditions on the fouling rate were investigated including: hot fluid Reynolds number (the dispersion), cold fluid Reynolds number, and time. The oil fouling rate was analyzed by determining the growth of fouling resistance with time and through pressure drop measurements. The influence of copper oxide (CuO) nanofluid on the fouling rate in the dispersion was also determined. It was found that the presence of dispersed oil causes a reduction in heat transfer coefficient by percentages depending on the Reynolds number of both cold and hot fluids and the concentration of oil. In addition, the time history of fouling resistance exhibited different trends with the flow rates of both fluids and its trend was influenced appreciably by the presence of CuO nanofluid.

An Experimental Investigation on the Influence of Copper Ageing on Flow Boiling in a Copper Microchannel

The study reports the experimental investigation on the influence of copper ageing on flow boiling heat transfer and pressure drop in copper microchannels of two different hydraulic diameters, 0.324 and 0.658 mm. Experiments have been conducted for different time intervals between the experimental runs. A reduction in the two-phase heat transfer coefficient was noticed with time, possibly due to the growth of oxidation layer. Energy-dispersive X-ray spectroscopy analyses of the fresh sample and the sample after the experiments indicated the growth of oxidation layer. Confocal microscope images and three-dimensional roughness profiles obtained using an optical profilometer also indicated a change in the surface texture. Deterioration in two-phase heat transfer coefficient was attributed to the increased surface wettability caused by the oxidation. The rate of reduction in heat transfer coefficient was higher for higher heat fluxes and smaller hydraulic diameter. A small increase in two-phase pressure drop was also noticed. Possible sources of oxidation were discussed in the study. The study underlines that the copper ageing factor be taken into account for the design and operation of two-phase microchannel heat sinks with copper substrate.

Pool boiling heat transfer of water and nanofluid outside the surface with higher roughness and different wettability

ABSTRACTIn order to investigate the effect of surface wettability on the pool boiling heat transfer, nucleate pool boiling experiments were conducted with deionized water and silica based nanofluid. A higher surface roughness value in the range of 3.9 ~ 6.0μm was tested. The contact angle was from 4.7° to 153°, and heat flux was from 30kW/m2 to 300kW/m2. Experimental results showed that hydrophilicity diminish the boiling heat transfer of silica nanofluid on the surfaces with higher roughness. As the increment of nanofluid mass concentration from 0.025% to 0.1%, a further reduction of heat transfer coefficient was observed. For the super hydrophobic surface with higher roughness (contact angle 153.0°), boiling heat transfer was enhanced at heat flux less than 93 kW/m2, and then the heat transfer degraded at higher heat flux.

Radiative and Viscous Ohmic Dissipation on MHD Tangent Hyperbolic Fluid over a Convectively Heated Sheet in a Suspension of Dust Particles

The present study deals with steady incompressible magneto hydrodynamic hyperbolic tangent fluid flow induced by a convectively heated stretching surface with the suspension of dust particles, Darcy-Forchheimer, thermal radiation, viscous dissipation and Ohmic heating. Similarity transformations were used to convert partial differential equations (PDEs) to a system of nonlinear ordinary differential equations (NODEs) which are solved numerically by Runge-Kutta Fehlberg method. The effect of pertinent parameters on velocity and temperature profiles of both fluid and dust phase within the boundary layer has been studied by considering various values of controlling parameters. Additionally, the skin friction coefficient and reduced heat transfer coefficient have been examined for various values of the governing parameters. It is found that Hartmann number and Forchheimer parameter reduce friction factor and heat transfer rates.

Early-age thermal analysis and strain monitoringof massive concrete structures

Hydration heat and thermal induced cracking have always been a fatal problem for massive concrete structures. In order to study a massive reinforced concrete wall of a storage tank for liquefied natural gas (LNG) during its construction, two mock-ups of 0.8 mx0.8 mx0.8 m without and with metal corrugated pipes were designed based on the actual wall construction plan. Temperature distribution and strain development of both mock-ups were measured and compared inside and on the surface of them. Meanwhile, time-dependent thermal and mechanical properties of the concrete were tested standardly and introduced into the finite-element (FE) software with a proposed hydration degree model. According to the comparison results, the FE simulation of temperature field agreed well with the measured data. Besides, the maximum temperature rise was slightly higher and the shrinkage was generally larger in the mock-up without pipes, indicating that corrugated pipes could reduce concrete temperature and decrease shrinkage of surrounding concrete. In addition, the cooling rate decreased approximately linearly with the reduction of heat transfer coefficient h, implying that a target cooling curve can be achieved by calculating a desired coefficient h. Moreover, the maximum cooling rate did not necessarily decrease with the extension of demoulding time. It is better to remove the formwork at least after 116 hours after concrete casting, which promises lower risk of thermal cracking of early-age concrete.

Optimization of the Heating Element in a Gas-Gas Heater Using an Integrated Analysis Model

Gas-gas heaters (GGHs) are used to reheat gases through desulfurization in coal-fired power plants to reduce low-temperature corrosion and white smoke. Wrinkled heating elements are installed inside the GGH to perform effective heat exchange. An optimization study of the heating element shape was conducted to reduce the differential pressure effectively and improve performance. An integrated analysis model was applied. Based on actual operational data, a computational fluid dynamic analysis was conducted on the L-type heating element and GGH system. The experiments applied the optimal latin hypercube sampling method, and numerical analysis was performed for each sample. Based on the response surface, the result of the sample was optimized through the pointer algorithm. For the integrated analysis model, validation was performed by comparison with the actual operational data, and the thermal-fluid characteristics of the heating element and GGH system were analyzed to set three parameters: plate angle, undulated angle, and pitch 1. From the optimization result, increases in the undulated angle and pitch 1 reduce the pressure drop by widening the heating element cross section. By increasing the plate angle, the heat transfer area is secured and the reduced heat transfer coefficient is compensated, improving the GGH performance.

Warming of Monolithic Structures in Winter

The present work attempts to develop a mathematical model for calculating the heat transfer coefficient of the fence of monolithic structures erected in winter. The urgency and, at the same time, the practical significance of the research lies in the fact that to date no simple, effective tool has been developed to ensure the elimination of the unfavorable thermally stressed state of a structure’s concrete from maximum equalization of temperatures across its cross-section. The main problem for concrete is a high temperature which leads to a sharp decrease in the quality of erected structures due to developing cracks. This paper based on the well-known Newton’s law and its differential equation demonstrates the formula of concrete cooling and the analysis of its proportionality coefficient. Based on the literature analysis, it is established that the proportionality coefficient is determined by the thermophysical properties of concrete, the size and shape of the structure, and the intensity of its heat exchange with the surrounding medium. A limitation was used on the temperature gradient over the section of the monolithic structure to derive a formula for calculating the reduced heat transfer coefficient of a concrete fence. All mathematical calculations are given for cooling monolithic constructions in the form of plates. At the end of the work an example is given for the calculation of the required reduced heat transfer coefficient for the fence ensuring compliance with the permissible concrete temperature gradient.

Effect of Convective Boundary Condition on MHD Carreau Dusty Fluid over a Stretching Sheet with Heat Source

The underlying intention of the present study is to analyze the steady incompressible magneto hydrodynamic Carreau Dusty fluid over a stretching sheet with exponentially decaying heat source. Convective conditions are considered to control the thermal boundary layer. Similarity transformations were used to convert partial differential equations (PDEs) to a system of nonlinear ordinary differential equations (NODEs) which are solved numerically by employing Runge-Kutta with Newton’s technique. The effect of pertinent parameters on velocity and temperature profiles of both fluid and dust phase within the boundary layer has been studied by considering various values of controlling parameters. In addition, skin friction coefficient and reduced heat transfer coefficient have been examined for various values of the governing parameters. It is observed that the rate of heat transfer depreciates with space dependent heat generation and enhanced with the existing convective condition.

Magnetohydrodynamic Cattaneo-Christov flow past a cone and a wedge with variable heat source/sink

In the present article, the problem of boundary layer flow of MHD electrically conducting fluid past a cone and a wedge with non-uniform heat source/sink along with Cattaneo-Christov heat flux is investigated numerically. At first, the flow equations are converted into ODE via appropriate self similarity transforms and the resulting equations are solved with the assistance of R.-K. and Newton’s methods. The influence of several dimensionless parameters on velocity and temperature fields in addition to the friction factor and reduced heat transfer coefficient has been examined with the support of graphs and numerical values. The heat transfer phenomenon in the flow caused by the cone is excessive when compared to the wedge flow. Also, the thermal and momentum boundary layers are not the same for the flow over a cone and wedge.

Theoretical analysis of heat transfer and friction factor for turbulent flow of nanofluids through pipes

AbstractA numerical model for determining the turbulent characteristics of fluid flow and heat transfer is presented, treating certain constants in the van Driest eddy diffusivity equation of momentum and heat as variables. The viscosity and thermal conductivity of nanofluids are estimated using regression equations. It was observed that the turbulent characteristics of nanofluids are different from those of water. The numerical results indicate a higher velocity of SiO2 nanofluid and lower eddy diffusivity compared to Cu under similar operating conditions. The nanofluid temperature gradient increases with concentration and decreases with temperature. However, the temperature gradient is significantly influenced by the particle density. Equations for estimating the coefficient and the Prandtl index in the eddy diffusivity equations of momentum and heat, respectively, are developed as a function of Reynolds number, concentration, and nanofluid properties. The Prandtl index value decreases with increasing concentration, reflecting the reduction in heat transfer coefficients observed at lower operating temperatures.

RESEARCH OF STATIONARY OPERATING MODES OF THERMOELECTRIC HEATEXCHANGE DEVICES

The necessity of detailed research, relevant consume cal semiconductor heat transfer devices. Distinctive features of thermoelectric devices flow type caused by the change of temperature of streams of coolant along the junctions of termoelectromotoare are identified. Differential equations systems determining the temperature change of the coolants, excluding from them the values of the temperatures of the junctions are used. A mathematical model for parallel flow and counter-flow thermoelectric heat pumps has been constuctucted. A mathematical model of a thermoelectric heat exchanger without any flows of heat in the gaps between the semiconductors has been developed. The cooled and heated liquids temperature dependences at the outlet of the heat exchanger from the heat transfer coefficient to the inner surface of the heat exchanger and reduced heat transfer coefficient from the outer surface of the heat exchanger have been constucted. It is proved that for the same values of heat transfer coefficient to the inner surface of the heat exchanger and reduced heat transfer coefficient from the outer surface of the heat exchanger reducing the temperature of fluid to be cooled significantly more than the increase in the temperature of the heated liquid. The main results obtained by the model are summarized. The analysis is carried out using MathCAD.

Investigation on the Effect of Type and Size of Nanoparticles and Surfactant on Pool Boiling Heat Transfer of Nanofluids

In our efforts to improve the pool boiling heat transfer of water, three sets of experiments are carried out to investigate the best coolant for heat removal among alumina, silica, and zinc oxide as nanoparticles and water as base fluid: (a) pool boiling heat transfer of γ-alumina/water nanofluid with and without surfactant in both distilled water and treated water as base fluids, (b) pool boiling heat transfer of silica/water nanofluid with two different nanoparticle sizes, and (c) pool boiling heat transfer of zinc oxide/water nanofluid with surfactant. In all the above experiments, the effect of heater surface on boiling heat transfer coefficient has been investigated by repeating the experiment using pure water on the coated surface before cleaning it. Moreover, two effective thermophysical properties of fluids, dynamic viscosity and surface tension, are measured to explain the boiling behavior of the nanofluids. The experimental results indicate that (a) the presence of γ-alumina in the base fluid enhances the pool boiling heat transfer coefficient, but sodium dodecyl sulphate (SDS) as surfactant deteriorates the performance of pool boiling heat transfer of γ-alumina/water nanofluid and (b) silica nanoparticles reduce the boiling performance of pure water. Moreover, the larger particle size of silica nanoparticles shows less reduction in heat transfer coefficient, (c) zinc oxide/water nanofluid is the best coolant among all the above-mentioned nanoparticles for heat removal.

Thermal Optimization of Window Frames

The thermal performance of window frames can easily be calculated using 2D numerical simulations. Several commercial software packages are available, and international standards provide a clear methodology to calculate the thermal transmittance. However, even though these methods are well known in academia and the research community in general, thermal optimization has not reached its full potential in the building industry yet and there is considerable margin for improvement. Specifically for small and medium size enterprises there is a lack of guidelines that are both generic enough to guarantee a wide-spread use, as well as specific enough to allow an easy and straightforward interpretation and implementation. In this research project, generic window sections were developed for vinyl, aluminium and wooden frames in collaboration with the building industry. Based on a market survey, typical approaches for improving the thermal performance for each type of window frame have been identified and described. Subsequently, the impact of separate improvements, as well as combined effects have been studied using both standardized and advanced calculation methods. For this, the heat transfer phenomena and the way these are modelled according to standard calculation procedures are discussed. Next to that, a number of secondary effect originating from standards are discussed, e.g. the thickness of the IGU, the depth of the window rebate, equivalent thermal conductivities and the impact of reduced heat transfer coefficients.

Development and Aerothermal Investigation of Integrated Combustor Vane Concept

This paper presents the development and aerothermal investigation of the integrated combustor vane concept for power generation gas turbines with individual can combustors. In this novel concept, first introduced in 2010, the conventional nozzle guide vanes (NGVs) are removed and flow turning is achieved by vanes that extend the combustor walls. The concept was developed using the in-house computational fluid dyanamics (CFD) code TBLOCK. Aerothermal experiments were conducted using a modular high-speed linear cascade, designed to model the flow at the combustor–vane interface. The facility is comprised of two can combustor transition ducts and either four conventional vanes (CVs) or two integrated vanes (IVs). The experimental study validates the linear CFD simulations of the IV development. Annular full-stage CFD simulations, used to evaluate aerodynamics, heat transfer, and stage efficiency, confirm the trends of the linear numerical and experimental results, and thus demonstrate the concept's potential for real gas turbine applications. Results show a reduction of the total pressure loss coefficient at the exit of the stator vanes by more than 25% due to a reduction in profile and endwall loss. Combined with an improved rotor performance demonstrated by unsteady stage simulations, these aerodynamic benefits result in a gain in stage efficiency of above 1%. A distinct reduction in heat transfer coefficient (HTC) levels on vane surfaces, on the order of 25–50%, and endwalls is observed and attributed to an altered state of boundary layer (BL) thickness. The development of IV's endwall- and leading edge (LE)-cooling geometry shows a superior surface coverage of cooling effectiveness, and the cooling requirements for the first vane are expected to be halved. Moreover, by halving the number of vanes, simplifying the design and eliminating the need for vane LE film cooling, manufacturing and development costs can be significantly reduced.

낮은 핀관의 형상이 LiBr 수용액의 풀 비등에 미치는 영향에 대한 실험적 연구

Abstract: P ool boiling heat transfer coefficients of a LiBr solution were obtained for seven low fin tubes having different fin pitch and fin height. The test range covered saturation pressure from 7.38kPa to 101.3kPa, heat flux from 20 kW/m 2 to 40 kW/m 2 and LiBr concentration from 0% to 50%. The optimum fin geometry for the present experimental range turned out to be 26 fpi with 0.18 mm fin height.The advantage of added heat transfer area and the disadvantage of slower bubble growth and departure appear to have yielded an optimum fin pitch. The heat transfer coefficient decreased as saturation pressure decreased and Libr concentration increased. The reason may be attributed to the low saturation pressure, which increased the bubble departure diameter and decreased the bubble departure frequency. As the LiBr concenreation increased, the saturation temperature increased and the mass diffusion rate decreased, which resulted in a reduced heat transfer coefficient. The heat transfer coefficients of the low fin tube were greater than those of the smooth tube. Correlations were developed based on the present data.

Effect of furnace pressure drop on heat transfer in a 135 MW CFB boiler

The effect of the pressure drop across the furnace on heat transfer inside large scale CFB boilers was investigated with a 1-D CFB combustion model developed at Tsinghua University. The results from the 1-D model agree well with the field test data carried out in a 135MWe CFB boiler. It was found that, at a certain load and with fixed size distribution of the coal particles fed to the boiler, a decrease in furnace pressure drop yields an increase in furnace temperature and reduced heat transfer coefficient. According to the heat balance, when the pressure drop decreases by 1kPa, the heat transfer coefficient is expected to decrease approximately by 2%. The increase in temperature with decreasing pressure drop is less steep, when the boiler load increases or smaller sized coal particles are fed to the boiler. In both cases, more fine particles are carried to the upper parts of the furnace which results in a more uniform solid volume fraction distribution along the furnace height. The influence of the pressure drop on heat transfer by changing the solid suspension density weakens. Operation of the bed at relatively low pressure drop by reducing the size of coal particles will ensure adequate heat transfer.

On the optimization of 3D-flow and heat transfer by using the Ice Formation Method: Vane endwall heat transfer

Technical flow devices are usually optimized by using numerical algorithms that analyze a multitude of design variants for the flow restraining geometry until an optimum solution is found. In this paper, we present an alternative approach: the Ice Formation Method (IFM). We show the application of this method to a turbine vane endwall in order to optimize the endwall with respect to heat transfer. On the basis of a flat endwall, we first created ice-contoured endwalls in our water channel test facility. We then analyzed these contours using numerical simulations with air to approve them for the working medium of gas turbines. Results show that the IFM reliably creates contours with reduced heat transfer coefficients (HTC). In this vein, we created one contour that reduces the average HTC by 5.2% compared to the baseline. Analyzing this contour revealed that it adapts to the flow field by breaking up the near endwall vortices and thus reducing secondary flows. We found that this reduction in HTC results from the natural character of the IFM to reduce entropy production rates due to heat conduction.

ЛОКАЛЬНИЙ КОЕФІЦІЄНТ ТЕПЛОВІДДАЧІ ПРИ КИПІННІ РОЗЧИНУ ІЗОБУТАН/ МІНЕРАЛЬНЕ МАСТИЛО В ТРУБІ

Experimental data for refrigerant R600a mixed with the compressor mineral naphthenic oil ISO VG 15 boiling in the smooth small-diameter (5.4 mm) tube are reported. The tests were conducted at the inlet pressure psuc in the range from 62.3 to 82.2 kPa, heat flux from 2754 to 4105 W/m2, mass velocity varied between 11.90 to 18.54 kg/(m2∙s) and oil concentrations at the inlet to the evaporator from 0.25 to 0.5 % by mass. The results of the research of the local heat transfer coefficient at the boiling of the isobutane/compressor oil solution flow are given in the paper. It is shown that the substantial reduction of heat transfer coefficient was observed at high concentrations of oil. In addition, during the boiling of the refrigerant/oil solution the heat transfer coefficient in the lower part of the pipe is decreasing with mass velocity increasing.