Demir, Fatih; Turgut, Oguz

Impingement jets are a promising cooling technique for achieving uniform and high heat transfer. This study investigates the heat transfer and flow fields of coaxial round and swirl jets on flat and finned impingement plates, both experimentally and numerically. Experimental and numerical studies are carried out under different parameters and states. Coaxial nozzle geometry, the dimensionless nozzle-to-plate distance (H/D = 0.8–3.2), fin length (0, 2, and 6 mm), flow rate ratio (Q*=0.176–0.823) and restricted and unrestricted jet depending on the presence or absence of blocking plate are taken into consideration as parameters and states. The total flow rate is constant at 85 L/min for all cases. The results show that uniform heat transfer on a flat impingement plate can be achieved for specific parameters. The average Nusselt number (Nu) increases with high Q* and low H/D. Moreover, changes in Nu with respect to variations in H/D are more pronounced at higher flow rate ratios. Although the impingement plates with fins increase the heat transfer, they negatively affect the uniformity of heat transfer on impingement plate. The formation and positioning of recirculation zones in the flow field are affected by the impingement plate geometry, fin length, H/D, and blocking plate. This study is important because it presents an innovation considering the combined effects of coaxial impinging jets and finned impingement plates on heat transfer and flow fields. High and uniform heat transfer is possible for certain parameters and states studied. These results can be guided in electronic device cooling and thermal management systems.

Onal, Busra Selenay; Kirkar, Safak Metin; Akgul, Dogan; Celen, Ali; Acikgoz, Ozgen; Dalkilic, Ahmet Selim; Kazi, Salim Newaz; Wongwises, Somchai

This paper presents a review on heat transfer coefficient and pressure drop during condensation and boiling in helically coiled tubes. The geometric parameters such as coil diameter, coil pitch and operating parameters containing saturation temperature, system pressure, mass flux, heat flux and vapor quality have significant effects on the heat transfer characteristics. The frictional pressure drop during flow boiling increases with increasing vapor quality and mass flux, and decreases with increasing system pressure. Also, the effect of the curvature on the two-phase frictional multiplier is negligible. The frictional pressure drop in condensation increased with increasing average vapor quality and mass flux, and decreased with increasing saturation temperature. According to the results reported, heat flux had little effect on the pressure drop during condensation. Flow boiling heat transfer coefficient in helical coils of large diameter showed the similar characteristics and trend with the straight tube as the system pressure decreased. Lockhart-Martinelli parameter was generally preferred to be used by the authors as a function in the correlations to predict flow boiling heat transfer coefficient. There was a decrease in the average Nusselt number with increasing heat flux for the constant Dean number during condensation. This reduction in the average Nusselt number indicated that the turbulence effect created by the secondary flow in the helically coiled tubes eliminated the effects caused by the difference in density. The correlations used for determination of Nusselt number and pressure drop in helical coils with their major findings are presented in details.

Khanmohammadi, Saber; Goodarzi, Mohsen; Khanmohammadi, Shoaib; Ganjehsarabi, Hadi

In this study, thermoeconomic optimization is applied to a modified transcritical (CO2) refrigeration cycle. Modified cycle includes an extraction of saturated vapor as a coolant stream from the separator feed to the intercooler. In order to perform multi criteria optimization two objective functions consist of cooling capacity and annual cost rate are chosen. Gas cooler temperature (Tgc), gas cooler pressure (Pgc), extraction mass flow rate (α) and evaporator temperature (Tev) are selected as decision variables based on parametric study. The thermodynamic and economic modeling of the system is performed. To determine analytical, objective functions GMDH (group method of data handling) neural network method is used and finally with a developed code in Matlab optimal values of decision variables have been obtained. In addition, to better understanding the change of decision variables in optimum point, the scatter distribution of these variables is carried out.