Experimental investigation of the effect vertical oscillation on the heat transfer coefficient of the finned tube

Assist. Prof. Dr. Saad M. Jalil

College of Engineering / University of Anbar

The aim of this work is to investigate experimentally the effect of the forced vibrations on the free convection heat transfer coefficient using heated longitudinally finned cylinder made of Aluminum. The effect of the vibration frequency ranged from 2 to16 Hz with various heat fluxes ranged from 500-1500 W/m2 . It was found that, the relation between the heat transfer coefficient and amplitude of vibration increased for all inclination angles from (0° - 45°), while the increment of inclination angle decreases the values of convection heat transfer coefficient. The results show that the heat transfer coefficient ratio (hv/ho) of longitudinal finned cylinders in 0 angle was (8%) and (30%) greater than those for the (30 ) and (45 o ) respectively.

Test rig description:

Two types of cylindrical aluminum finned hot surfaces were used in this experiment which are triangular and rectangular longitudinal fins. The total length of the heated surface is (320 mm), the finned length is (300 mm), the outer diameter is 48 mm at the finned end. The height of the fin is 13 mm. Eight triangular and rectangular longitudinal fin equally distributed by (45) in both models were used. A heater is placed inside the hot surface internal diameter and heat flux is
varied. Fig. (1a) shows the two heated surfaces. Fig. shows that the holder is U-shaped and made from aluminum substance to hold the weight exerted on vibrator. The holder base is also made of aluminum. The hot surfaces are fixed to connection arms using Teflon substances to reduce the heating loss at its ends. The angle of the hot surface can be changed from θ=0°, to 45° using a lever mechanism which connects the holder and its base. Thereby, the measured vibration amplitude would be approximately the same for both the holder and the heated cylinder. The electric circuit includes a heating coil power of 1000W. Eight thermocouples were installed at the heated surface and distributed uniformly. Ambient temperature is measured by using a thermocouple which is placed 15cm away from the hot surface.

The test was carried out first by heating the hot surface and ensures steady state conditions were achieved after 4 hours. Then temperature reading was recorded with no vibration. Once temperature readings were obtained, the vibration effect was applied and the same procedure to achieve steady state and record the temperature were performed and vibration amplitude was recorded as well. The applied vibration ranged between 2 to 16 Hz.

Conclusions
This study analyzed and investigated the effect of vibrations which occurring in the radiator system and heat exchangers, including the external vibration effect on the heat transfer coefficient. Based on theexperimental results in the previous section, the effect of the vertical oscillation on the heat transfer coefficient is illustrated. The vibration intensity affected on the heat transfer coefficient ratio (hv/ho) of horizontal angle 0 at the model 1, where its values increased by 8%and 30% compared with angle 30 and 45, respectively. Also the heat transfer coefficient (hv/ho) values of longitudinal finned cylinders in angle 0 increased 1.5% compared with the angle 30o and 13.5% at the angle 45o at model 2. From all the results in this study, the significant effects of external vibration on fins cylinder performance and its effects on the heat transfer coefficient should be considered