Hong-Sen Kou

Flow Reversal and Heat Transfer of Fully Developed Mixed Convection in Vertical Channels

The present analysis is concerned with flow reversal phenomena and heat transfer characteristics of the fully developed laminar combined free and forced convection in the heated vertical channels. Three fundamental combinations of thermal boundary conditions on the respective wall surface (namely isoflux-isoflux, isofluxisothermal, and isothermal-isothermal) are considered separately so as to investigate extensively their distinct influence on the flow pattern. Results of the velocity distribution and temperature distribution as well as the Nusselt number in terms of bulk mean temperature are carried out. Based on the analytical solutions obtained, flow reversal adjacent to the relatively colder wall is found to exist within the channel as Re/Gr is below than a threshold value, which is related to the thermal boundary conditions. Parameter zones for the occurrence of reversed flow are presented. Comparisons and verification are made using the existing numerical solutions at locations far downstream of developing flow.

Effect of the directional blood flow on thermal dose distribution during thermal therapy: an application of a Green's function based on the porous model.

This study presents the effects of directional blood flow and heating schemes on the distributions of temperature and thermal dose during thermal therapy. In this study, a transient bioheat transfer equation based on the porous medium property is proposed to encompass the directional effect of blood flow. A Greens function is used to obtain the temperature distribution for this modified bioheat transfer equation, and the thermal dose equivalence is used to evaluate the heating results for a set of given parameters. A 10 x 10 x 10 mm3 tumour tissue is heated by different heating schemes to investigate the thermal dose variation with the clinical therapeutic arrangement. For a rapid heating scheme, the domain of thermal lesion can effectively cover the desired therapeutic region. However, this domain of thermal lesion may extend to the downstream normal tissue if the porosity is high and the averaged blood velocity has a larger value.

Effect of effective tissue conductivity on thermal dose distributions of living tissue with directional blood flow during thermal therapy

Abstract This study proposes a modified transient bioheat transfer equation based on combing the porous medium property and the scalar effective thermal conductivity equation in order to include the directional effect of blood flow. By applying the porous medium model to describe the collective behavior of the heat transfer in living tissue with many small blood vessels, an analytical solution can be obtained by Greens function. Simulation results quantitatively show that the blood perfusion rate, the averaged blood velocity, the porosity and the heating period are the crucial factors determining the distribution of thermal dose for thermal therapies. In addition, longer heating schemes induce dependencies of both the blood perfusion and the enhanced thermal conductivity on increased temperature.

The impact of thermally significant blood vessels in perfused tumor tissue on thermal dose distributions during thermal therapies

This study demonstrates the effects of thermally significant blood vessels on thermal lesion sizes during thermal therapies. The thermal model combines the Pennes bio-heat transfer describing for perfused tissue and the energy equation for blood vessel. A finite difference method was used to solve the transient equation of heat transfer in perfused tumor tissue with a blood vessel in cylindrical coordinates. The thermal dose distribution was determined by the formulation proposed by Sapareto and Dewey. Simulation results quantitatively show that the blood vessel diameter and the averaged blood velocity are the crucial factors determining the thermal dose distribution near the blood vessel for thermal therapies. When a blood vessel diameter passing throughout the heated volume is larger than 1 mm, the thermal dose level may become too low and then lose its hyperthermia effect

Combined boundary and inertia effects for fully developed mixed convection in a vertical channel embedded in porous media

Abstract The fully developed laminar mixed convection in a vertical channel embedded in porous media has been solved by using non-Darcy flow model. Through proper manipulation of nondimensional variables and parameters, the governing equations are derived and the thermal boundary conditions on the left and right walls can be prescribed as isothermal-isothermal, isothermal-isoflux, or isoflux-isothermal, respectively. Including the Darcian force, buoyancy force and boundary effect, the exact solutions for temperature and velocity profiles are obtained. Meanwhile, parametric zones for the occurrence of flow reversal based on the analytical solutions are presented. Finally, the numerical solution is also provided to investigate their further influence due to the existence of inertia effect.

Fully developed laminar mixed convection through a vertical annular duct filled with porous media

The fully developed laminar mixed convection through a vertical annular duct embedded in a porous medium has been solved by using the non-Darcian flow model, where thermal boundary conditions on inner and outer walls are prescribed as isothermal-isothermal, isothermal-isoflux, and isoflux-isothermal, separately. The analytical solution has been derived to obtain velocity and temperature profiles, mass flow rate, wall friction factor and heat carried out by fluid. Finally, the parametric zones for flow characteristics of velocity distribution with the upward or downward flow are demonstrated.

COOLING EFFECTIVENESS OF CUTTING FLUID IN CREEP FEED GRINDING

In this study, the heat transfer paths among the grinding fluid, the grains and the workpiece are investigated for the creep feed grinding. As heat enters the workpiece, the majority of heat is carried away by the fluid. Thus, the cooling effectiveness of the grinding fluid is defined and calculated through the application of the numerical method to illustrate how much the fraction of the heat is carried away by the grinding fluid. The results reveal that water has higher cooling effectiveness than oil. In addition, the cooling effect of the grinding fluid becomes more significant at lower workpiece speed, higher grinding depth and greater wheel speed.

EFFECT OF LONGITUDINAL SEPARATOR SHEET CONDUCTION ON THE TRANSIENT THERMAL RESPONSE OF CROSSFLOW HEAT EXCHANGERS WITH NEITHER GAS MIXED

ABSTRACT The present study investigates the transient temperature behavior of a direct transfer type, single-pass, crossflow heat exchanger with neither gas mixed, accounting for the two-dimensional longitudinal conduction effect on the separator sheet. Numerical results show the variation of the mean exit temperatures of hot and cold gases. The approaching time to steady state is also presented to describe the influence of longitudinal conduction and number of transfer units. Finally, comparisons among step, ramp, and exponential responses are demonstrated to display thermal performance deterioration due to the wall conduction effect.

THE EFFECT OF LONGITUDINAL WALL CONDUCTION IN A THREE-FLUID CROSSFLOW HEAT EXCHANGER

Abstract Because of the effect of longitudinal wall conduction on the thermal performance in a direct transfer-type, single-pass, three-fluid crossflow heat exchanger, the exit mean temperatures and the deterioration factors in each fluid are calculated by the numerical method. Unit construction of the three-fluid crossflow heat exchanger is that fluid 2 is sandwiched between fluid I and fluid 3. The results of deterioration factor versus number of transfer units and the inlet temperature of fluid 3 are explored to demonstrate the influences of different heat capacity rate ratios, heat transfer resistance ratios, and longitudinal wall conduction ratios

Thermal performance of crossflow heat exchanger with nonuniform inlet temperatures

The thermal performance of a direct transfer type, single pass, crossflow heat exchanger, which includes two hot streams with different temperature levels on one side and one cold stream on the other, is investigated with the numerical method. By changing the inlet temperature distribution of the two hot streams or discarding one hot stream which has lower temperature, the exit mean temperature of the cold stream can be calculated. The results show the region which is suitable for utilizing two hot streams or for abandoning one lower hot stream to earn the higher heat recovery effectiveness. Finally, the present study demonstrates that the effects of nonuniform inlet temperatures in the crossflow heat exchanger bring about the increase in the rates of thermal performance.