Showing papers in "Journal of Heat Transfer-transactions of The Asme in 2018"
135 citations
TL;DR: In this paper, the authors present a review of basic gas turbine heat transfer issues with advanced cooling technologies and documents important relevant relevant references for future research references, and propose a turbine blade cooling system with efficient cooling strategies under new harsh working environments.
Abstract: Gas turbines have been extensively used for aircraft engine propulsion, land-based power generation, and industrial applications. Power output and thermal efficiency of gas turbines increase with increasing turbine rotor inlet temperatures (RIT). Currently, advanced gas turbines operate at turbine RIT around 1700 °C far higher than the yielding point of the blade material temperature about 1200 °C. Therefore, turbine rotor blades need to be cooled by 3–5% of high-pressure compressor air around 700 °C. To design an efficient turbine blade cooling system, it is critical to have a thorough understanding of gas turbine heat transfer characteristics within complex three-dimensional (3D) unsteady high-turbulence flow conditions. Moreover, recent research trend focuses on aircraft gas turbines that operate at even higher RIT up to 2000 °C with a limited amount of cooling air, and land-based power generation gas turbines (including 300–400 MW combined cycles with 60% efficiency) burn alternative syngas fuels with higher heat load to turbine components. It is important to understand gas turbine heat transfer problems with efficient cooling strategies under new harsh working environments. Advanced cooling technology and durable thermal barrier coatings (TBCs) play most critical roles for development of new-generation high-efficiency gas turbines with near-zero emissions for safe and long-life operation. This paper reviews basic gas turbine heat transfer issues with advanced cooling technologies and documents important relevant papers for future research references.
80 citations
57 citations
TL;DR: Patrick E. Hopkins et al. as discussed by the authors studied the Steady-state temperature rise during laser heating of multilayer thin films in optical pump-probe techniques and found that the temperature increased with the number of layers.
Abstract: Patrick E. Hopkins Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, VA 22904; Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904; Department of Physics, University of Virginia, Charlottesville, VA 22904 e-mail: phopkins@virginia.edu On the Steady-State Temperature Rise During Laser Heating of Multilayer Thin Films in Optical Pump–Probe Techniques
50 citations
TL;DR: In this article, the performance of mesh wick heat pipe (HP) using TiO2/H2O nanofluid (0.5, 1.0, and 1.5 vol %) as working fluid for different (50, 100, and 150 W) power input were investigated.
Abstract: The enhancements in thermal performance of mesh wick heat pipe (HP) using TiO2/H2O nanofluid (0.5, 1.0, and 1.5 vol %) as working fluid for different (50, 100, and 150 W) power input were investigated. Results showed maximum 17.2% reduction in thermal resistance and maximum 13.4% enhancement in thermal efficiency of HP using 1.0 vol % nanofluid as compared to water. The wick surface of the HP was then coated with TiO2 nanoparticles by physical vapor deposition method. The experimental investigation had been also carried out on coated wick HP using water as working fluid. Results showed 12.1% reduction in thermal resistance and 11.9% enhancement in thermal efficiency of the HP as compared to uncoated wick HP using water. Temporal deteriorations in thermal performance during prolonged working (2, 4, and 6 months) of HP were also studied. Temporal deterioration in thermal performance of HP filled with nanofluid depends upon the deterioration in thermophysical properties of nanofluids. The deterioration is due to the agglomeration and sedimentation of nanoparticles with respect to the time. Comparative study shows that after a certain time of operation, thermal performance of HP with nanoparticle coated wick superseded that of the HP filled with nanofluid. Therefore, nanoparticle coating might be a good substitute for nanofluid to avoid the stability issues. The present paper provides incentives for further research to develop nanofluids that avoid the encountered sedimentation or agglomeration.
46 citations
TL;DR: In this paper, a nonlinear mixed convection flow due to Riga plate with double stratification is considered and heat transfer analysis is reported for heat generation/absorption and nonlinear thermal radiation.
Abstract: This paper addresses nonlinear mixed convection flow due to Riga plate with double stratification. Heat transfer analysis is reported for heat generation/absorption and nonlinear thermal radiation. Physical problem is mathematically modeled and nonlinear system of partial differential equations is achieved. Transformations are then utilized to obtain nonlinear system of ordinary differential equations. Homotopic technique is utilized for the solution procedure. Graphical descriptions for velocity, temperature, and concentration distributions are captured and argued for several set of physical variables. Features of skin friction and Nusselt and Sherwood numbers are also illustrated. Our computed results indicate that the attributes of radiation and temperature ratio variables enhance the temperature distribution. Moreover, the influence of buoyancy ratio and modified Hartmann number has revers effects on rate of heat transfer.
42 citations
40 citations
TL;DR: In this article, the mass transfer coefficient from bulk flows to pore surfaces in chemically reactive flows for both ordered and disordered porous structures is analyzed for both cylinders in a staggered and a line arrangement.
Abstract: We present lattice Boltzmann (LB) simulations for the mass transfer coefficient from bulk flows to pore surfaces in chemically reactive flows for both ordered and disordered porous structures. The ordered porous structure under consideration consists of cylinders in a staggered arrangement and in a line arrangement, while the disordered one is composed of randomly placed cylinders. Results show that the ordered porous structure of staggered cylinders exhibits a larger mass transfer coefficient than ordered porous structure of inline cylinders does. It is also found that in the disordered porous structures, the Sherwood number (Sh) increases linearly with Reynolds number (Re) at the creeping flow regime; the Sh and Re exhibit a one-half power law dependence at the inertial flow regime. Meanwhile, for Schmidt number (Sc) between 1 and 10, the Sh is proportional to Sc; for Sc between 10 and 100, the Sh is proportional to Sc. [DOI: 10.1115/1.4038555]
39 citations
38 citations
37 citations
TL;DR: In this article, the effect of short branches on the thermal conductivity of a polyethylene (PE) chain was studied. And the results showed that the branch has a positive effect to decrease thermal conductivities of a PE chain.
Abstract: In this paper, we have studied the effect of short branches on the thermal conductivity of a polyethylene (PE) chain. With a reverse non-equilibrium molecular dynamics method applied, thermal conductivities of the pristine PE chain and the PE-ethyl chain are simulated and compared. It shows that the branch has a positive effect to decrease the thermal conductivity of a PE chain. The thermal conductivity of the PE-ethyl chain decreases with the number density increase of the ethyl branches, until the density becomes larger than about 8 ethyl per 200 segments, where the thermal conductivity saturates to be only about 40% that of a pristine PE chain. Because of different weights, different types of branching chains will cause a different decrease of thermal conductivities, and a heavy branch will leads to a lower thermal conductivity than a light one. This study is expected to provide some fundamental guidance to obtain a polymer with a quite low thermal conductivity.
TL;DR: In this article, a novel approach for the mitigation of flow maldistribution problem in parallel MCHS has been proposed using variable width microchannels, which facilitated in effective uniform cooling across the entire projected area of MCHs.
Abstract: The problem of flow maldistribution is very critical in microchannel heat sinks (MCHS). It induces temperature nonuniformity, which may ultimately lead to the breakdown of associated system. In the present communication, a novel approach for the mitigation of
flow maldistribution problem in parallel MCHS has been proposed using variable width microchannels. Numerical simulation of copper made parallel MCHS consisting of 25 channels has been carried out for the conventional design (CD) and the proposed design
(PD). It is observed that the PD reduces flow maldistribution by 93.7%, which facilitated in effective uniform cooling across the entire projected area of MCHS. Temperature fluctuation at fluid–solid interface is reduced by 4.3 C, whereas maximum and average temperatures of microchannels projected area are reduced by 2.3 C and 1.1 C, respectively. PD is suitable in alleviating flow maldistribution problem for the extended range of off design conditions.
TL;DR: In this article, the thermal behavior of DBD barrier discharge actuators under different flow conditions was analyzed and the authors concluded that the temperature distribution is not influenced by the dielectric thickness, but it changes when the actuator operates under an external flow.
Abstract: Dielectric barrier discharge (DBD) plasma actuators have several applications within the field of active flow control. Separation control, wake control, aircraft noise reduction, modification of velocity fluctuations, or boundary layer control are just some examples of their applications. They present several attractive features such as their simple construction, very low mass, fast response, low power consumption, and robustness. Besides their aerodynamic applications, these devices have also possible applications within the field of heat transfer, for example film cooling applications or ice formation prevention. However, due to the extremely high electric fields in the plasma region and consequent impossibility of applying classic intrusive techniques, there is a relative lack of information about DBDs thermal characteristics. In an attempt to overcome this scenario, this work describes the thermal behavior of DBD plasma actuators under different flow conditions. Infra-red thermography measurements were performed in order to obtain the temperature distribution of the dielectric layer and also of the exposed electrode. During this work, we analyzed DBD plasma actuators with different dielectric thicknesses and also with different dielectric materials, whose thermal behavior is reported for the first time. The results allowed to conclude that the temperature distribution is not influenced by the dielectric thickness, but it changes when the actuator operates under an external flow. We also verified that, although in quiescent conditions the exposed electrode temperature is higher than the plasma region temperature, the main heat energy dissipation occurs in the dielectric, more specifically in the plasma formation region.
TL;DR: In this paper, the sharp corners of the triangular cross-sectional duct are modified by converting it into circular shape, and the curvature radius of rounded corners for different models was kept constant (0.33 times the duct height).
Abstract: Turbulent flow heat transfer and friction penalty in triangular cross-sectional duct is studied in the present paper. The sharp corners of the duct are modified by converting it into circular shape. Five different models were designed and fabricated. Heat transfer through all the models was investigated and compared conventional triangular duct under similar conditions. The curvature radius of rounded corners for different models was kept constant (0.33 times the duct height). The numerical simulations were also performed and the obtained result validated with the experimental findings and close match observed between them. The velocity and temperature distribution is analyzed at particular location in the different models. Because of rounded corners, higher velocity is observed inside the duct (except corners) compared to conventional duct. Considerable increase in Nusselt number is seen in model-5, model-4, model-3, and model-2 by 191%, 41%, 19%, and 8% in comparison to model-1, respectively, at higher Reynolds number (i.e., 17,500). But, frictional penalty through the model-5, model-4, model-3, and model-2 increased by 287%, 54%, 18%, and 12%, respectively, in comparison to model-1 at lower Reynolds number (i.e., 3600).
TL;DR: In this article, six groups of nanowire arrayed surfaces with various structures and sizes were fabricated to enhance spray cooling by improving the surface wettability and the liquid transport to quickly rewet the surface and avoid dry out.
Abstract: Enhancing spray cooling with surface structures is a common, effective approach for high heat flux thermal management to guarantee the reliability of many high-power, high-speed electronics and to improve the efficiency of new energy systems. However, the fundamental heat transfer enhancement mechanisms are not well understood especially for nanostructures. Here, we fabricated six groups of nanowire arrayed surfaces with various structures and sizes that show for the first time how these nanostructures enhance the spray cooling by improving the surface wettability and the liquid transport to quickly rewet the surface and avoid dry out. These insights into the nanostructure spray cooling heat transfer enhancement mechanisms are combined with microstructure heat transfer mechanism in integrated microstructure and nanostructure hybrid surface that further enhances the spray cooling heat transfer.
TL;DR: In this paper, a parametric study was carried out to investigate the effects of various pin-fin configurations on the flow and heat transfer characteristics of micro-pin-fin heat sinks.
Abstract: An experimental study is performed to investigate water flow and heat transfer characteristics in silicon micro-pin-fin heat sinks with various pin–fin configurations and a conventional microchannel, with a length of 25 mm, a width of 2.4 mm, and a height of 0.11 mm. The micro-pin-fin heat sinks have different fin arrangements, fin shapes, and fin pitches. The results show that the micro-pin-fin heat sinks have the better overall thermal-hydraulic performance including the heat transfer enhancement and the pressure drop penalty compared to the conventional microchannel. A parametric study is carried out to investigate the effects of various pin-fin configurations on the flow and heat transfer characteristics. The linear relationship between fRe and Re is found for the water flow through the micro-pin-fin heat sinks for the first time. A new friction factor correlation is further developed based on the linear relationship between fRe and Re. Taking the effects of the various pin-fin configurations on the Nusselt number into consideration, a new Nusselt number correlation is also developed. The new correlations of friction factor and Nusselt number predict the experimental data well. An infrared thermo-imaging system was used to measure the temperature field of water heat transfer in the micro-pin-fin heat sinks and the conventional microchannel. The infrared thermo-images show the more uniform temperature profile in the transverse direction for the micro-pin-fin heat sinks than that for the conventional microchannel, which indicates the better heat transfer performance of the former than the latter. The dominant mechanism of heat transfer enhancement caused by the micro-pin-fins is the hydrodynamic effects, including fluid disturbance as well as the breakage and re-initialization of the thermal boundary layer near the wall of the heat sinks.
TL;DR: In this article, a three-dimensional numerical model has been built and validated to parametrically investigate the thermal behavior of transcritical n-Decane in mini cooling channels with microribs under near critical pressure.
Abstract: Microrib is a very promising heat transfer enhancement method for the design of scramjet regenerative cooling channels. In this paper, a three-dimensional numerical model has been built and validated to parametrically investigate the thermal behavior of transcritical n-Decane in mini cooling channels with microribs under near critical pressure. The results have shown that the height and pitch of microrib perform a nonmonotonic effect on the convective heat transfer coefficient of n-Decane inside the cooling channel and the optimal microrib parameters stay at low values due to dramatic changes of coolant thermophysical properties in the near critical region. Due to severe thermal stratification and near critical conditions, there will be a significant recirculation zone in vertical direction near microrib, and its interaction with the strong secondary flow in axial direction caused by limited channel width of mini-channel will largely enhance the local convective heat transfer and its downstream region. Besides, the dramatically changing thermophysical properties of n-Decane will lead to a locally remarkable heat transfer enhancement phenomenon similar to impingement cooling at the front edge of microribs.
TL;DR: In this paper, a scaling analysis of the point heat source in steady state on a semi-infinite solid is presented, where all characteristic values related to an isotherm can be reduced to a dimensionless expression dependent only on the Rykalin number (Ry).
Abstract: This paper presents a systematic scaling analysis of the point heat source in steady state on a semi-infinite solid. It is shown that all characteristic values related to an isotherm can be reduced to a dimensionless expression dependent only on the Rykalin number (Ry). The maximum width of an isotherm and its location are determined for the first time in explicit form for the whole range of Ry, with an error below 2% from the exact solution. The methodology employed involves normalization, dimensional analysis, asymptotic analysis, and blending techniques.The expressions developed can be calculated using a handheld calculator or a basic spreadsheet to estimate, for example, the width of a weld or the size of zone affected by the heat source in a number of processes. These expressions are also useful to verify numerical models.
TL;DR: In this article, a series of functionally graded metal foam matrices of the same mass (i.e., the same solidity) were examined in view of their heat transfer performances.
Abstract: Fully developed forced convective heat transfer within a channel filled with a functionally graded metal foam matrix was investigated analytically for the case of constant wall heat flux. A series of functionally graded metal foam matrices of the same mass (i.e., the same solidity) were examined in views of their heat transfer performances. The porosity either increases or decreases toward the heated wall following a parabolic function. Among the metal foam matrices of the same mass, the maximum heat transfer coefficient exists for the case in which the porosity decreases toward the heated wall (i.e., more metal near the wall). The heat transfer coefficients in such channels filled with a functionally graded metal foam matrix are found 20–50% higher than that expected from the increase in the effective thermal conductivity. Hence, functionally graded metal foam matrices are quite effective to achieve substantially high heat transfer coefficient with an acceptable increase in pressure drop.