Electronic Cooling

Undoubtedly that increasing the turbine inlet temperature is an important requirement to increase the gas turbine efficiency. There are external and internal efficient cooling methods were used to protect the blade surface from the failure. The internal cooling was enhanced by using rib, pin fins, impingement and swirl techniques. In a swirl cooling, the swirling motion is created by tangential inlet velocity that enhances the heat transfer compared to axial inlet velocity.
More researchers studied the effect of swirling technique on heat transfer coefficient at different Reynolds numbers [1]. In this study four turbulence models of standard ?-? model, RNG ?-? model, standard ?-? model and the SST ?-? model were used to validate with the experiment. The validation results showed that SST ?-? model gave a good agreement with the experiment for velocity near the wall and Nusselt number through the chamber circumferential.
The numerical results showed that heat transfer enhances in swirl chamber with increasing Reynolds numbers. The authors interpreted an increasing of heat transfer coefficient due to development in the thermal boundary layer and vortices interaction and increases this effect with increasing Reynolds numbers and in addition to increase the advection and the flow became more turbulent, more distortion and diffusion [2]. Also at increasing Reynolds numbers, a large pressure drop and large circumferential velocity generated [2].

Another study to decrease the pressure drop in swirling chamber was performed experimentally and analytically [3]. In this study two outlets were used instead of single outlet. The pressure drop decreased for case of two outlets because of decreasing the vortex strength. At increasing Reynolds numbers, the circumferential velocity and the pressure drop increased.
The effect of five inlet jets with one outlet swirling chamber on the Nusslet number and pressure drop was performed [4]. The results showed that the Nusslet number and the pressure drop through the chamber for single inlet jet were higher than the case of five jets in case of fixed mass flow rate.
Swirling action effect on turbulence intensity was investigated for single inlet and exit [5-6]. The results showed that the turbulence increases with swirling motion at inlet and decay along the chamber with decreasing the swirl intensity.
The effect of changing the exit orifice on swirling motion was studied [7-9]. This study used three different exit geometry; large orifice, small orifice and eccentric small orifice. The results showed that different helical flow structures generated near the wall and around the centerline. Also the swirling number didn’t change significantly with these geometries and with increasing Reynolds numbers.
Different swirl directions and number of film cooling holes effects on adiabatic film cooling effectiveness were investigated [10-11]. The results showed that the film cooling effectiveness can be increased or can be decreased depending on these conditions.
Comparison study between impingement cooling and swirl cooling over Re of 7500 to 12500 was investigated [12]. They concluded that the swirl cooling and impingement cooling matched at high mass flow rate moreover the heat transfer distribution in the swirl cooling is more uniform in the axial direction.
Another study investigated the augmentation of heat transfer by swirl cooling techniques in slot shaped channel with aspect ratio of 3:1 and jets issuing from side walls [13-15]. They used different configurations to enhance the swirl flow in the channel.
The effect of flow redirection on the heat transfer by using three different outlet geometries; straight, tangential, 180° bend outlets was investigated [16]. The results showed that the heat transfer was higher by four times compared to an axial flow, but without significant changes in heat transfer for three swirl cases.
The double swirl chambers with single and multiple inlet jets and with different swirl directions are another method for augmentation the heat transfer [17-19]. By using a proper swirl direction, vortex strength increased leading to increase the heat transfer coefficient.
Four turbulence models of standard k??, Realizable k??, standard k?? and SST k?? were used to validate the experimental data of averaged Nusslet number [19]. The validation results showed that SST k?? model was good agreement with the experiment.

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