Heat and Mass Transfer

A one-dimensional heat conduction equation with time- and temperature-dependent heat sources was employed to study the steady-state and transient response of a composite superconductor subjected to a thermal disturbance. An integral formulation was used to solve the steady-state problem of current redistribution and heat generation. The results of the integral formulation are compared with those of an analytical solution. The two solutions agree with each other except when the analytical solution fails as the temperature in the superconductor begins to exceed the critical temperature. Transient solutions were obtained by the finite-difference technique and the results are compared with a known analytical solution. Results of numerical calculations of the transient response of a composite superconductor subjected to an initial pulsed disturbance are presented. It is demonstrated that the superconductor can switch between the superconducting and the current-sharing state. The transient response and the stability of the composite conductor depend on the magnitude and duration of the disturbance, the dimensionless temperature θ*, and the dimensionless parameter φ.

An apparent thermal conductivity for inhomogeneous materials is widely used. In this paper it is demonstrated that the apparent thermal conductivity for stationary heat conduction is not sufficient to describe the transient heat response of an inhomogeneous medium. In the geometry we used the heat transfer is estimated too high when the stationary thermal conductivity is employed. A numerical solution of the equation of thermal diffusion has been used to check several approximations. For short and for long times a separate approximate analytic expression can be used.

Im ersten Teil dieser Untersuchung wird zur Betrachtung des dynamischen Verhaltens instationärer Temperaturfelder in den Wandungen zylindrischer Rohre ein mathematisches Modell erstellt und mit Hilfe der Laplace-Transformation ausgewertet. Im einzelnen werden dabei die Übertragungsfunktionen der Rohrwandtemperaturen hergeleitet und für den Fall der Abweichung vom stationären Zustand unter dem Einfluß äußerer Störungen explizit dargestellt. Im zweiten Teil der Untersuchung wird das sich daraus ergebende dynamische Verhalten der Wandtemperatur fluiddurchströmter Rohre für einige Beispiele in Form von Ortskurven dargestellt.

Stall is a common flow phenomenon in the rotating machinery under partial load conditions. The stall phenomenon can seriously affect the operation efficiency and stability of the machinery. In the present research, the stall phenomenon in a centrifugal pump is numerically studied using the SST k-ω turbulence model. In the present work, four different flow rates (1.0 Qd, 0.7 Qd, 0.5 Qd and 0.3 Qd, where Qd is the design flow rate) are investigated, and results reveal that with the decreasing of flow rate, the stall can be divided into the preliminary stall and stationary stall according to the flow structure. When the flow rate decreases to 0.5, the vortexes become strong, but not occupy the whole passage, which is defined as the preliminary stall. When the flow rate further decreases to 0.3 Qd, a fully developed stationary stall appears. Under this condition, the periodic process of stationary stall can be classified into four stages: incepting stage, developing stage, shedding stage and decaying stage. The dominant frequencies of pressure fluctuations under stationary stall conditions are fi, and the maximum amplitudes of pressure fluctuations of PS4 and PS5 at 0.3 Qd are about 5 times that at 1.0 Qd due to the trailing edge vortexes at the blade outlet.

Superparamagnetic ferrites are potential materials explored for magnetic nanoparticle hyperthermia. Theranostics utilizing infrared thermography with iron oxide nanoparticles demand better heating characteristics and thermal sensitivity that are affected by the biological fluid and absorption by the tissue layer. Agar gel phantoms were incorporated with the ferrite particles dispersed in albumin and water, where the analyses present improved sensitivity with Mn-doped Fe3O4 compared to magnetite. Mn-doping shows attenuation in temperature difference from 6.7K to about 2K with layer thickness up to 2mm. The Lambert absorption coefficient determined for the phantom layers from the subsurface sensing is 0.07cm− 1 with the Mn-doped particles both in water and albumin. A prudent investigation of the absorbance characteristics of phantom layers suggests hyperthermia with long-wave infrared thermography (8–13μm) as a promising combination for the theranostics of subcutaneous tumours.

Laser-induced cavitation bubble behavior on the solid walls of different materials was experimentally investigated by focusing the pulsed laser on the wall. To examine the influence of wall reflectivity on laser-induced cavitation, high-speed observations of the bubble behavior and impulsive-force measurements were carried out on aluminum, steel, copper, and acrylic resin walls. It was found that the bubble behaviors and impulsive forces vary with the wall materials, and their behaviors depend on the wall reflectivity. Increased bubble diameter, bubble collapse time, and high impulsive force were observed on the walls with high reflectivity. This result can be due to the increased laser illumination on the highly reflective walls. This promotes the heating of liquid near the wall at the laser breakdown in the first impact on the wall, while the increased bubble diameter by this effect promotes the bubble collapse phenomenon and intensive shockwave formation, resulting in the high impulsive forces on the wall in the second impact.

In this study, a three-dimensional computational fluid dynamics model has been developed for an anode-supported planar SOFC. The conservation equations of mass, momentum, species/charges and thermal energy are solved by finite volume method for a complete unit cell consisting of 13 parallel channels in both anode and cathode. The simulation results of the developed model are well in agreement with the experimental data obtained at same conditions. In this study, the co-flow arrangement with hydrogen utilization of 60 % and operating voltage of 0.7 V is used as the base case, and compared with the counter-flow arrangement. The predicted results reveals that the maximum temperature obtained in the counter-flow arrangement is about 10 °C lower than that of co-flow, but the counter-flow arrangement has a higher temperature gradient between the respective anodes and cathodes in a cross-section normal to the main flow direction, especially in the air inlet region of the cell (x = 0.04 m),which is very harmful to the lifetime of materials. The current density is very unevenly distributed along and normal to the flow direction for both the co- and counter-flow arrangements, and the maximum values occur at junctions of the electrodes, channels and ribs, which causes higher over-potentials and ohmic heating.

This paper reports an experimental study of free convection heat transfer from rectangular fin-arrays on a horizontal base. An experimental set-up was constructed and calibrated, 15 sets of fin-arrays and a base plate without fins were tested in atmosphere. Fin height was varied from 6 mm to 26 mm, fin spacing was varied from 6.2 mm to 83 mm. The base-to-ambient temperature difference was also varied independently and systematically with the power supply to heater ranging from 8 W to 50 W. Fin length and fin thicknesses were fixed at 100 mm and 3 mm, respectively. The experimental program was conducted so as to clearly delineate the separate roles of fin height, fin spacing and base-to-ambient temperature difference. It was found that for a given base-to-ambient temperature difference the convection heat transfer rate from fin-arrays takes on a maximum value as a function of fin spacing and fin height. For a given base-to-ambient temperature difference the enhancement of the convection heat transfer rate of fin-arrays relative to that for base plate without fins is strongly dependent on the fin spacing to fin height ratio and number of fins. A correlation was also presented relating the convection heat transfer rate of fin-arrays relative to that for base plate without fins with the relevant non-dimensional parameters.