When we evaluate complicated cooling heat transfer characteristics of high-temperature steel with water, a solution of the inverse heat conduction problem (IHCP) is essential to specify the surface heat flux and surface temperature change with time from the measurement of the inside solid temperatures. In the present study, the analytical solution of the IHCP with the Laplace transform technique proposed by Monde is applied to steel cooling processes. This technique deals uniform and constant thermal properties of the solid, and the measured temperature change at two depths is approximated with half-power polynomials of time. The steel cooling processes show a very high cooling rate and a large temperature drop after quenching. Therefore, the temperature dependence of thermophysical properties could not be negligible, and much better accuracy of the approximate functions is also required for the complicated measured temperature history. We applied modifications to the analysis, such as a simple and explicit treatment of the temperature-depending properties and superimposing approximate functions. The accuracy of the modified technique was assessed for quenching temperature histories in a steel specimen. From the sensibility of approximation parameters constructing the superimposed functions, the optimum parameters were specified for the measured temperature of quenching experiments. The explicit treatment of the thermal properties ensures accurate estimation results for a higher cooling rate, in which the product of the temperature gradient near the surface and the temperature gradient of the thermal conductivity is small.
One of the improvement techniques of effective thermal conductivity for metal hydride particles bed is a insertion of radial fins into the bed, by which the heat transfer characteristics and reaction kinetics can be enhanced. Rectangular fins are inserted from 3 to 6 into the metal hydride particles bed and hydrogen absorption amount of metal hydride has been measured by constant volume Sievert's method. Reaction kinetics is defined as a ratio of 80% of maximum hydrogen absorption over the absorption time when the amount of hydrogen absorption reaches its corresponding value. As the results, radial fins are useful for the enhancement of the heat transfer and reaction kinetics of metal hydride particles bed. Reaction kinetics of reactor with 6 fins can become two times faster than that without the fin.
A previous procedure to measure thermal diffusivity and thermal conductivity simultaneously using an inverse solution for one-dimensional unsteady heat conduction can be simplified by improving the inverse solution with moving window clearing the issue related to settlement of time left in previous one. The new procedure does not need any time when the temperature change starts at a sensor position and can simply choose a time duration during which the measured temperature change becomes larger than the required accurate temperature change. The measurement is usually completed within about 1 minute until the temperature rise at the thermocouple position reaches a certain temperature level which makes its error level lower than a couple of percents. This method has merit being independent of surface condition except for the requirement of two or three sensing positions in the material. The accuracy of the estimated values is also similar to the error level of sensor at the position.
In the present study, we focused on the rapid liquid heating process and the subsequent boiling explosion that occurs when a liquid jet comes in contact with a very hot surface during jet impingement quenching. Assuming the liquid jet as 1-D semi-infinite solid during its brief contact with the surface, a model has been proposed based on the ideas of 1-D heat conduction and homogeneous nucleation. In this model, a liquid control volume having the size of a critical cluster at the boundary is considered and the corresponding energy balance is obtained by accounting for the two parallel competing processes that takes place inside the liquid control volume, namely, transient external heat deposition and internal heat consumption due to liquid superheat, bubble nucleation and subsequent growth. Results obtained are presented in terms of the liquid temperature escalation within the control volume, the limit of maximum attainable liquid temperature and the time necessary to reach the temperature limit at the boiling explosion. The boiling explosion condition as defined in the present model is also compared with the theoretical boiling explosion condition denoted by the upper bound of evaporative heat flux across the liquid-vapor interface, qmax,max. The time duration of the solid-liquid contact prior to the boiling explosion at different surface temperatures as obtained by the proposed model may be helpful for better understanding the possible surface temperature oscillations due to repetitive solid-liquid contact in the first few microseconds of jet impingement quenching.
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