Effect Of Internal Heat Transfer Research Articles

Combined silicon carbide and zirconia open cell foams for the process intensification of catalytic methane combustion in lean conditions: Impact on heat and mass transfer

For catalytic process intensification, a series of open cell foams (OCFs) made of silicon carbide (SiC) and zirconia (Zir) with pore density of 30 ppi coated with 3 wt% PdO/Co3O4 as catalyst were combined together and tested toward methane oxidation in lean conditions. In each combination, the SiC OCF was positioned in the reactor on the inlet side of the reactant gases followed by the Zir OCF. The reactor was fed at different weight hourly space velocities (WHSV, 30 and 90 NL h−1 gcat−1) and inlet methane concentrations (0.5 and 1 vol%). The best results are obtained with the combination where two supports of same length but with different thermal conductivity (higher at the inlet of the reactor, SiC, and lower at the outlet, Zir) are used in series. For all OCF combinations, mass transfer effects were evaluated using the characteristic resistances (kinetic, internal and external mass transfer). The external and internal heat transfer effects were analyzed using the Mears and Anderson criteria. Furthermore, a comparison in terms of volumetric heat transfer coefficient and heats of removal/reaction was performed.

Heat-Transfer-Corrected Isothermal Model for Devolatilization of Thermally Thick Biomass Particles

An isothermal model is commonly used in computational fluid dynamics (CFD) modeling of biomass devolatilization in fluidized beds. However, the particle internal heat transfer, which is neglected by the isothermal model, influences significantly the devolatilization process for large biomass particles. To consider the effect of internal heat transfer, a heat-transfer-corrected isothermal model is introduced by comparing the differences between an isothermal model and a nonisothermal model. Two correction coefficients for external heat transfer, HT, and reaction rate, HR,i, were defined to correct the conventional isothermal model. The predictions of the heat-transfer-corrected isothermal model and the nonisothermal model were in good agreement with the experimental data for both thermally thick (Biot number, Bi ≥ 1.0) and thermally thin (Bi < 1.0) biomass particles, while the conventional isothermal model gave reasonable results only for thermally thin biomass particles. The heat-transfer-corrected isothermal model was further implemented in a CFD model to simulate biomass devolatilization in a batch bubbling fluidized bed. Compared to the conventional isothermal model, the heat-transfer-corrected isothermal model had similar computational efficiency, but it predicted a lower heating rate and a lower devolatilization rate, which were in good agreement with the observations from single-particle modeling.

Improved Heat Transfer Correction Method for Turbocharger Compressor Performance Measurements

The compressor adiabatic performance map is important for turbocharger matching with engine. Its accuracy was affected by the heat transfer phenomenon in the turbocharger. The corrected method for compressor performance measurement which affected by heat transfer was widely investigated by some authors. The aim of this work was to measure compressor adiabatic efficiency by simplified method, and estimated the effects of internal heat transfer on compressor efficiency. This approach only added five temperature sensors on the turbocharger based on the standard instruments and doesn’t need other special instruments, and the heat transfer properties of turbocharger and compressor adiabatic efficiency could be calculated by lumped model. This method was validated by other experiments. Finally, the heat absorbed by compressor under different operating conditions was analyzed by this method. The results showed water-cooling was main influenced factor, and water-cooling was a barrier for heat transfer from turbine to compressor.

An experimental investigation of internal heat transfer in an automotive turbocharger compressor

An experimental investigation developed on a small turbocharger compressor for automotive application is presented. The study focuses on the effects of internal heat transfer on compressor efficiency, evaluated for different values of inlet pressure, mass flow rate and compressor rotational speed. Infrared thermography has been used to evaluate the heat transfer rate from the turbine to the compressor and to correct the measured compressor diabatic efficiency. The approach has been validated by performing additional measurements under quasi-adiabatic conditions, avoiding map distortion due to heat transfer. Simple relationships for the prediction of internal heat transfer, validated against the measured values, have been proposed. The practical significance of the results, with reference to turbocharger compressor performance, is outlined.

Aerothermal Challenges in Syngas, Hydrogen-Fired, and Oxyfuel Turbines—Part II: Effects of Internal Heat Transfer

Future advanced turbine systems for electric power generation, based on coal-gasified fuels with CO2 capture and sequestration, are aimed for achieving higher cycle efficiency and near-zero emission. The most promising operating cycles being developed are hydrogen-fired cycle and oxyfuel cycle. Both cycles will likely have turbine working fluids significantly different from that of conventional air-based gas turbines. In addition, the oxyfuel cycle will have a turbine inlet temperature target at approximately 2030 K (1760°C), significantly higher than the current level. This suggests that aerothermal control and cooling will play a critical role in realizing our nation’s future fossil power generation systems. This paper provides a computational analysis in comparing the internal cooling performance of a double-wall or skin-cooled airfoil to that of an equivalent serpentine-cooled airfoil. The present results reveal that the double-wall or skin-cooled approach produces superior performance than the conventional serpentine designs. This is particularly effective for the oxyfuel turbine with elevated turbine inlet temperatures. The effects of coolant-side internal heat transfer coefficient on the airfoil metal temperature in both hydrogen-fired and oxyfuel turbines are evaluated. The contribution of thermal barrier coatings toward overall thermal protection for turbine airfoil cooled under these two different cooling configurations is also assessed.

Thermodynamic analysis of different two-stage transcritical carbon dioxide cycles

The aim of this paper is the thermodynamic evaluation and optimisation of different two-stage transcritical carbon dioxide cycles. Five different cycles are studied: basic single-stage cycle, single-throttling with two-stage compression cycle, split cycle, phase separation cycle and single-stage cycle coupled with a gas cooling circuit. Each basic cycle is analysed for the effect of internal heat transfer between different streams of refrigerants. In the case of two-stage compression, intermediate cooling between the compressor stages is present. An analysis on the Plank cycle for intermediate pressure higher than critical one is performed. Each cycle is optimised with regards to energy performance, calculating the optimal values of both the upper and the intermediate pressures. In the case of split cycle, the ratio of the mass flow rate in the main stream to the one in the auxiliary stream is also optimised.

Similarity solution for laser-driven shock waves in a dust-laden gas with internal heat transfer effects

The non-adiabatic flow behind a laser-driven strong shock wave propagating in a mixture of gas and small solid particles is the subject of this paper. A similarity solution which accounts for the influence of internal heat fluxes due to high temperatures achieved at the centre has been obtained. The heat fluxes in the blast-wave equations are considered in terms of Fourier's law for conduction and by an expression for thermal radiation of the diffusion type. As for adiabatic flow, it is assumed that the equilibrium-flow condition is maintained and that the variable laser energy is completely absorbed at the shock front according to a time-dependent power law. The formulation results in a two-point boundary-value problem. The effects of a parameter characterising the various energy input of the blast wave on the similarity solution as well as on its limits have been examined. The computations have been performed for various values of mass concentration of the solid particles and for the ratio of density of solid particles to the constant initial density of gas.

Effects of internal heat transfer and preferential diffusion on stretched spray flames

The extinction of dilute spray flames propagating in a stagnation-point flow under the influence of flow stretch, preferential diffusion, and internal heat transfer is analyzed using activation energy asymptotics. A completely prevaporized mode and a partially prevaporized mode of flame propagation are identified. The internal heat transfer, associated with the liquid fuel loading and the initial droplet size of the spray, provides heat loss and heat gain for rich and lean sprays, respectively. It is found that the flow stretch coupled with Lewis number ( Le) reduces and enhances the burning intensity of the lean methanol-spray flame ( Le>1) and rich methanol-spray flame ( Le<1), respectively; and that the flame extinction characterized by a C-shaped curve for the Le>1 flame is dominated by the flow stretch, while the S-shaped extinction curve for the Le<1 flame is mainly influenced by the internal heat loss associated with the droplet gasification process.

Quasi-similar solutions for blast waves with internal heat transfer effects

The point explosion problem with internal heat transfer effects taken into account is analysed. The classical inviscid solutions to this problem yield a non-physical phenomenon of infinite temperature and zero density at the center of explosion for all times. With heat transfer fluxes considered, the solution near the center of symmetry is improved and finite values for temperature are obtained. The non-self-similar solution of the problem is based on the quasi-similar approximate technique which reduces the non-linear partial differential equations governing the problem to ordinary differential ones. However, this formulation yields a two-point boundary-value problem. To facilitate the integration, the flow field is first divided into two regions: an outer inviscid region and an inner region where dissipation effects are manifested. This results in two sets of ordinary differential equations expressing the conservation equations in the inner and outer regions which are then solved and matched together to yield the composite solution. Secondly, the problem is then transformed into an initial-value one. Using the results of the composite solution, the governing equations can be integrated directly from the center until the shock front. The structure of the non-self-similar flow fields with internal heat transfer effects is then fully determined for specified values of the heat transfer parameters.

Perturbation solutions for blast waves with convective and radiative heat transfer

The non-self-similar point explosion problem is investigated, taking into accont internal heat transfer effects. The conductive heat flux is expressed in terms of Fourier's law and radiation is considered to be of the diffusion type.

A Universal Model for the Overall Thermal Conductivity of Porous Media

A general field theory is developed in this work to estimate the overall degradation of the thermal conductivity of the solid due to the presence of internal cavities. In addition to the volume fraction of cavities in the matrix material, the effect of internal heat transfer across the pore surface on the overall thermal conductivity is absorbed in an added tensor. Based on the field theory thus formulated, application is made to investigate the effects of pore geometry and internal heat transfer on the overall thermal conductivity. In the aspect of pore geometry, the overall thermal conductivity of the porous medium is obtained for insulated spherical cavities and penny-shaped cracks. In the aspect of internal heat transfer, on the other hand, the effect of heat conduction and convection across the pore surface is analyzed. All the cases under consideration are compared with the experimental results and the dominant physical modulus in the overall thermal conductivity is identified for each case.

The effect of internal heat transfer in cavities on the overall thermal conductivity

Based on an analytical model, degradation of the overall thermal conductivity is studied in this paper to account for the internal mechanism of heat transfer between the solid medium and the fluid inside cavities. The cavity is assumed to randomly distribute in the solid with its density measured by the volume fraction. The model is capable of characterizing the effect of internal heat transfer such as conductive and convective heat transfer across the surfaces of cavities. In the case involving the convection mode, the Biot number has been identified to be the modulus dominating the overall thermal conductivity, while in the case involving the conduction mode, the ratio of the thermal conductivity of the solid medium to that of the fluid inside cavities is the major influencing factor. The effect of interfacial conduction and convection is found to be pronounced when the volume fraction of pores in the solid exceeds 15%. The large deviation from the experimental result by considering insulated cavities is redeemed by considering heat balance of heat conduction or convection across the cavity surface.

Effects of internal heat transfer on the structure of self-similar blast waves

Profiles of gasdynamic parameters in self-similar blast waves, taking into account the influence of conduction and radiation fluxes due to high temperatures attained at the centre, are determined. In the blast-wave equations these fluxes are expressed in terms of the Fourier law for heat conduction and a differential expression for radiative transport in a semi-grey gas model. Various boundary conditions are considered in order to account for different ways in which blast waves are initiated and driven. Similarity requirements are implemented in the solution by compatible functional forms of gas conductivity and absorptivity, as well as the opacity of the shock front. This formulation yields a two-point boundary-value problem, which is then transformed into an initial-value problem in order to facilitate the integration. As a particular example, a detailed solution for the constant-energy case is obtained, covering the whole range of relative heat-transfer effects expressed in terms of radiative to gasdynamic energy fluxes, from the adiabatic flow field, on one extreme, to the isothermal, on the other.

Effectiveness of Radiation as a Structural Cooling Technique for Hypersonic Vehicles

The effectiveness of radiation as a structural cooling technique is examined for vehicles which are subject to large variations in heating over their surface. An analysis is made of the more significant effects of internal heat transfer within idealized structures, both with and without external insulation on areas of greatest aerodynamic heating intensity. Selective use of insulation leads to structural temperature reductions substantially greater than those obtained without insulation. The results are presented in nondimensional form, and numerical examples illustrate applications to hypersonic flight vehicles.