Balkrishna Mehta

Effect of Periodic Pulsations on Heat Transfer in Simultaneously Developing Laminar Flows: A Numerical Study

Heat transfer in the channels and ducts are well understood in the steady laminar flows for engineering applications. In contrast, unsteady flows have potential for research as many aspects of such flows are still unclear. Periodic pulsating flow in a channel is a kind of unsteady flow which requires further investigation because (i) many upcoming applications, especially in mini-micro scale engineering domain e.g. enhanced mixing, MEMS applications, bio-fluidic devices and thermal management of electronics etc. (ii) critical review of literature reveals that there is prevailing confusion related to the species transport coefficients. Thus, need for a systematic parametric study, both numerical and experimental, cannot be overemphasized. In this paper, two different configurations of laminar pulsatile internal flow, i.e. Case (i): unidirectional flow with axial superimposed pulsations (flow in circular axisymmetric tube) and, Case (ii) unidirectional flow with superimposed transverse pulsations (parallel plates) have been numerically scrutinized. Effect of frequency (Womersley number, Wo), Prandtl number (Pr), Reynolds number (Re) and amplitude ratio, on the instantaneous and time averaged heat transfer and friction factor (Poiseuille number) is studied. It is found that the change in species transport is either marginal or highly limited and is primarily occurring in the developing length of the channel/ plate. Nusselt number under pulsating conditions in the fully developed flow regime is not very different from its steady counterpart. Enhancement of species transport due to such periodic pulsatile internal flows, over and above the non-pulsatile regular flow conditions, is questionable, and at best, rather limited. Enhancement in heat transfer is seen in Case (ii) under certain operating conditions. This latter configuration is more attractive than the former and further optimization studies are required to improve understanding.Copyright

Solar Updraft Tower—A Potential for Future Renewable Power Generation: A Computational Analysis

The full-scale three-dimensional analysis of solar updraft tower power plant in Manzanares, Spain has been performed using commercially available CFD tool ANSYS Fluent. The two-equation k-\(\varepsilon \) turbulent model with standard wall function has been utilized for the fluid flow. The soil has been modeled with the consideration of the fact that temperature at 20 m depth remains constant throughout the year. The surface-to-surface radiation model is also included in the heat transfer model. The simulation has been performed for the steady state without/with radiation, the transient state with/without thermal storage on 8th of June. In the present simulation, water has been taken as thermal storage. It has been found that results improve considerably by including radiation effect, closely match with the results published from the plant. There is a reduction in the maximum velocity with the thermal storage; however, sufficient energy is available in thermal storage to overcome intermittency of insolation. The strong dependency of the plant on insolation can be reduced with the thermal storage.

Local Experimental Heat Transfer of Single-Phase Pulsating Flow in a Square Mini-Channel

Disturbing the flow with a particular pulsating frequency alters the thermal and hydrodynamic boundary layer thus affecting the inter-particle momentum and energy exchange. Due to this enhanced mixing, augmentation in the heat transfer is expected. Obviously, the parameters like pulsating frequency vis-a-vis viscous time scales and the imposed pulsating amplitude will play an important role in the enhancement of the heat transfer. Several numerical heat transfer and fluid flow studies on pulsating flows have been reported in the literature but the conclusions are not coherent. Lack of experimental study in hydrodynamics as well as in heat transfer of laminar pulsating flows attracts to revisit this problem especially, in mini-channels. Technological developments in measurement and instrumentation have enabled to experimentally investigate the thermo-hydrodynamic study of laminar pulsating flows in mini-channels as an augmentation technique for heat transfer. In this work, we have undertaken the experimental measurements of heat transfer of single-phase laminar pulsating flow in square mini-channel of cross-section 3 mm × 3 mm. The study is done at two different pulsating frequency 0.05Hz and 1Hz (Womersley number, Wo = 0.8 and 3.4 respectively). These two values are chosen because velocity profile exhibits different characteristic for Wo > 1 (annular effect, i.e., peak velocity near the wall) and Wo < 1 (conventional parabolic profile). The heat transfer study has been done in a square channel of made on polycarbonate sheet with one side heating. Heater (made of SS, 70 microns thin strip, negligible thermal inertia) itself is one of the walls of the square channel making constant heat flux thermal condition and its instantaneous temperature is measured by using pre-calibrated InfraRed camera. Fluid bulk mean temperature has been determined by energy balance and one K-type thermocouple is also placed in the fluid at the outlet cross-section. By using these temporal data, space averaged instantaneous Nusselt number has been obtained. It is observed that for measured frequency range, the overall enhancement in the heat transfer is not attractive for laminar pulsating flow in comparison to steady flow of same time-averaged flow Reynolds number. It is found that the change in species transport is either marginal or highly limited and is primarily occurring in the developing length of the channel/ plate. Enhancement of species transport due to such periodic pulsatile internal flows, over and above the non-pulsatile regular flow conditions, is questionable, and at best, rather limited.Copyright