Journal of Thermophysics and Heat Transfer最新文献

An analytical expression is developed for transient thermal spreading resistance from an isothermal circular source in a cylindrical flux tube as a function of constriction ratio and time. The flux tube is semi-infinite. The spreading resistance expression is obtained from the temperature expression by solving the heat equation. For short times, the dimensionless transient spreading resistance is proportional to dimensionless time based on the square root of the source area. For long times, the dimensionless spreading resistance approaches the values of the corresponding steady-state expression in the literature. For small constriction ratios, dimensionless spreading resistance approaches the classic isothermal half-space limit. A numerical analysis is presented which shows excellent agreement with the analytical solution. Approximate correlations for dimensionless resistance are also presented for both the isothermal and the isoflux cases.

Plasma generation in hypersonic flows is analyzed using a two-temperature model of nonequilibrium air. The uncertainties in electron number density predictions are assessed for flow scenarios that correspond to both strongly shocked and strongly expanded flows, and the dependencies of the calculated uncertainties on individual input parameters are quantified. Ionization levels behind 5 and 7 km/s normal shocks are found to be most sensitive to the associative ionization reactions producing ${\mathrm{O}}_2^{+}$ and ${\mathrm{NO}}^{+}$ in the region of peak electron number density, with nitric oxide kinetics dominating the uncertainty downstream. The higher levels of ionization behind a 9 km/s shock are found to strongly depend on the electron impact ionization of atomic nitrogen as well as the charge exchange between ${\mathrm{N}}_2^{+}$ and N. Recombining flow scenarios depend on many of the same processes that influence the shocked flows, with the notable addition of the reassociation reaction ${\mathrm{O}}^{+}+{\mathrm{N}}_2\leftrightarrow {\mathrm{NO}}^{+}+\mathrm{N}$, which is responsible for large uncertainties in electron number density in net recombining flows. The results provide valuable insight into the typical magnitude of uncertainty associated with plasma formation predictions in hypersonic flows and identify the parameters that should be targeted in efforts to reduce those uncertainties.

The present numerical study explores the flow physics and heat transfer characteristics around three unconfined inline horizontally placed cylinders arranged in a vertical array subjected to ambient air. It investigates the effect of Rayleigh number (i.e., 10² ≤ Ra ≤ 10⁶) and the center-to-center spacing (1≤ S/D ≤ 9). The study shows that the augmentation or reduction of heat transfer rate from an individual cylinder in the array to that of a single unconfined cylinder under identical conditions strongly depends on the separation distance and Rayleigh number. Moreover, the average Nu from the bottom cylinder deteriorates only at very close spacing, whereas after S/D ≥ 3, it attains an asymptotic value to that of a single cylinder. The heat transfer rates from the middle and top cylinders relative to a single cylinder also deteriorate drastically at close spacing. In contrast, a slight enhancement is observed at relatively large spacing and higher Ra.

This study presents a method for estimating the space-dependent thermal contact resistance between the two-layer walls of a furnace using the boundary element method (BEM) and conjugate gradient method (CGM) for the heat conduction problem. The global solution equation in matrix form is derived using the interface conditions, and the BEM is used to solve the direct problem. The CGM minimizes the objective function and calculates the sensitivity coefficients with the complex variable derivation method (CVDM). Comparative results show that the present approach is more accurate, stable, and efficient than the conventional CGM, which is attributed to the calculation of the sensitivity coefficients by CVDM. The effects of the value of thermal contact resistance, thermal conductivity ratio, Biot number, initial guess, measurement error, and the number and position of measurement points on the inversion results are also analyzed. Finally, the effectiveness of this approach is demonstrated through numerical examples, and the inversion results show its stability, efficiency, and accuracy in identifying different and complex distributions of thermal contact resistance. Furthermore, this approach is feasible for nonintrusive measurement, which is very meaningful in practical applications.

A material response solver that predicts the response of charring ablative materials under different degrees of physical modeling complexity is developed. The solver provides a versatile environment for engineering analyses and incorporates a third-party library for the evaluation of thermodynamic/transport properties of pyrolysis gas mixture and chemical kinetics, if necessary. Thermal nonequilibrium between the solid and the gas phases is considered using the two-equation model. A novel reactor network approach is used for modeling pyrolysis gas flow inside the porous ablative material, allowing simulations with various gas compositions and reaction mechanisms. Effects of chemical and thermal nonequilibrium and influences of the porosity and permeability of the porous structure on the response of charring ablative materials are explored. It is observed that higher porosity and smaller permeability values induce local thermal equilibrium, and chemical reactions increase the temperature differences between the phases.