Sompol Skullong

Experimental Investigation on Turbulent Convection in Solar Air Heater Channel Fitted with Delta Winglet Vortex Generator

Abstract The paper presents an experimental study on the heat transfer and flow friction characteristics in a solar air heater channel fitted with delta-winglet type vortex generators (DWs). The experiments are conducted by varying the airflow rate for Reynolds number in the range of 5000 to 24000 in the test section with a uniform heat-flux applied on the upper channel wall. Firstly, the DW pairs are mounted only at the entrance of the lower wall of the test channel (called DW-E) to create multiple vortex flows at the entry. The effect of two transverse pitches ( R P = P t / H = 1 and 2) at three attack angles ( α = 30°, 45° and 60°) of the DW-E with its relative height, b/H = 0.5 (half height of channel) is examined. Secondly, the 30° DWs with three different relative heights ( b/H = 0.3, 0.4 and 0.5) are placed on the upper wall only (absorber plate, called DW-A) of the test channel. The experimental result reveals that in the first case, the 60° DW-E at R P = 1 provides the highest heat transfer and friction factor while the 30° DW-E at R P = 1 performs overall better than the others. In the second case, the 30° DW-A at b/H = 0.5 yields the highest heat transfer and friction factor but the best thermal performance is found at b/H = 0.4.

Thermal characteristics in square channel with 45° staggered baffle inserts

The paper presents a study of heat transfer in a heat exchanger channel inserted with staggered inclined baffles. The channel has a square section with uniform wall heat flux conditions. The fluid flow and heat transfer characteristics are presented for Reynolds numbers based on the hydraulic diameter of the channel ranging from 4000 to 25,000. The inclined baffles with an axial pitch equal to three times of channel height and with the attack angle of 45° are mounted in tandem and staggered arrangement on the lower and upper walls of the test channel. Effects of four baffle-to-channel height ratios (e/H = 0.1, 0.2, 0.3 and 0.4) on heat transfer in terms of Nusselt number and pressure loss in the form of friction factor are experimentally investigated. The experimental result shows that the insertion of staggered inclined baffles with the e/H=0.4 provides higher heat transfer and friction factor values than others. This is caused by higher baffle-to-channel height ratios of using e/H = 0.4 interrupting the flow and diverting its direction thus promoting high levels of mixing over others.

Thermal Behaviors in a Square Duct with U-Ribbed Tape Inserts

This paper presents an experimental study on thermal characteristics in a constant heat-fluxed square-duct heat exchanger with U-ribbed tape inserts. The experiments are carried out by varying airflow rate for Reynolds number ranging from 4000 to 38,000. The insertion of the U-ribbed tape is performed with an axial rib-pitch set to four times duct-height (4H) at a single attack angle, α=45° and the ribbed tape is diagonally inserted in the square duct in order to generate longitudinal vortex flows. Effects of five blockage ratios (e/H=0.1, 0.15, 0.2, 0.25 and 0.3) on heat transfer and friction loss are experimentally investigated. The experimental result shows that the insertion of the U-ribbed tape at e/H=0.3 provides the highest heat transfer and friction factor values but the one at e/H=0.25 yields the highest thermal performance enhancement.

Heat Transfer Enhancement in a Solar Air Heater Channel with Discrete V-Baffles

This article presents the study of heat transfer enhancement in a uniform heat-fluxed channel fitted with discrete V-shaped baffles. The experiments are carried out by varying airflow rate for Reynolds number ranging from 4100 to 22,000. The V-baffles with relative height ratio, e/H = 0.15 and the attack angle, α = 45o, are mounted repeatedly on the upper plate only, similar to an absorber plate of solar air heater systems. The effects of four baffle-pitch to channel-height ratios (PR= 0.5, 1.0, 1.5 and 2.0) on heat transfer in terms of Nusselt number and pressure loss in the form of friction factor are experimentally investigated. The experimental results show that the use of the discrete V-baffles leads to a considerable increase in Nusselt number and friction factor in comparison to the smooth channel alone. The V-baffled channel with PR=0.5 provides the highest heat transfer, friction factor and thermal enhancement factor.

Heat Transfer in Round Tube with Rectangular-Winglet Vortex Generators

Effect of 30° rectangular-winglet vortex generators (WVGs) mounted in the central core of a circular tube on convection heat transfer and friction loss is experimentally investigated in the present work. The rectangular-WVGs with two different winglet-height to tube-diameter ratios (called blockage ratio, BR = b/D = 0.1 and 0.2) and three winglet-pitch to tube-diameter ratios (PR=P/D=0.5, 1.0, and 1.5) are introduced. In the experiment, air at ambient condition is passed through the uniform heat-fluxed circular tube for Reynolds numbers (Re) in a range of 500024,000. The use of WVGs is to generate longitudinal vortex flows in the tube. The experimental results of heat transfer and pressure loss presented in terms of Nusselt number and friction factor are compared between the inserted and the smooth tubes. It is found that the BR and PR provide a significant effect on the thermal performance of the test tube. The results reveal that at smaller PR, the WVG with BR=0.2 provides the highest heat transfer and friction factor but the one with BR=0.2, PR=1.5 yields the best thermal performance.