Darryl E. Metzger Memorial Session Paper: Experimental Heat Transfer and Friction in Channels Roughened With Angled, V-Shaped, and Discrete Ribs on Two Opposite Walls

Abstract

Experimental investigations have shown that the enhancement in heat transfer coefficients for air flow in a channel roughened with angled ribs is on the average higher than that roughened with 90 deg ribs of the same geometry. Secondary flows generated by the angled ribs are believed to be responsible for these higher heat transfer coefficients. These secondary flows also create a spanwise variation in heat transfer coefficient on the roughened wall with high levels of heat transfer coefficient at one end of the rib and low levels at the other end. In an effort basically to double the area of high heat transfer coefficients, the angled rib is broken at the center to form a V-shaped rib, and tests are conducted to investigate the resulting heat transfer coefficients and friction factors. Three different square rib geometries, corresponding to blockage ratios of 0.083, 0.125, and 0.167, with a fixed pitch-to-height ratio of 10, mounted on two opposite walls of a square channel in a staggered configuration, are tested in a stationary channel for 5000 < Re < 30,000. Heat transfer coefficients, friction factors, and thermal performances are compared with those of 90 deg, 45 deg, and discrete angled ribs. The V-shaped ribs are tested for both pointing upstream and downstream of the main flow. Test results show that: (a) 90 deg ribs represent the lowest thermal performance, based on the same pumping power, and is essentially the same for the 2:1 change in blockage ratio, (b) low-blockage-ratio (e/Dh = 0.083) V-shaped ribs pointing downstream produced the highest heat transfer enhancement and friction factors. Among all other geometries with blockage ratios of 0.125 and 0.167, 45 deg ribs showed the highest heat transfer enhancements with friction factors less than those of V-shaped ribs, (c) thermal performance of 45 deg ribs and the lowest blockage discrete ribs are among the highest of the geometries tested in this investigation, and (d) discrete angled ribs, although inferior to 45 deg and V-shaped ribs, produce much higher heat transfer coefficients and lower friction factors compared to 90 deg ribs.

Effect of Alterations in V-down Rib on the Performance of an Artificially Roughened Solar Air Heater Duct

“…This enhancement is primarily due to the refreshment of the air moving along the rib (secondary flow) through the gaps near the trailing ends of the V-rib. Taslim et al (1996) observed that, heat transfer from the heated surfaces roughened with V-down ribs is low near the trailing ends, due to heating up of the air as it moves along the ribs from leading to trailing ends. Aharwal et al (2009) (2011) also concluded that introduction of a gap near the trailing end of the inclined and Vshaped ribs respectively, enhances the rate of heat transfer from the absorber plate of a solar air heater.…”

“…Figure 5a) shows the flow disturbances caused by the application of continuous V-down rib. As explained by Taslim et al (1996) and Lei et al (2012), application of V-down rib causes a portion of the flow to move along the rib (called secondary flow) in the shape of counter-rotating vortices towards the apex of the V-rib. Counter-rotating vortices coming from two leading edges of the V-rib interfere with each other near the apex and a portion of them move toward the depth of the duct to form a pair of vertical loops, while the remaining portion jumps the rib along with the primary flow.…”

“…Large amount of literature is available on the use of ribs in various patterns for the enhancement of heat transfer (Han et al, 1991;Taslim et al, 1996;Karwa, 2003). Initial studies (Han et al, 1978;Gupta et al, 1993) used ribs positioned at 90° to the direction of flow for periodic interruption of the primary flow and reported enhancement in the heat transfer with considerable pressure drops.…”

“…With the aim to further enhancing the rate of heat removal with lesser pressure drop penalty, studies were performed using inclined ribs (Han and Park, 1988;Park et al, 1992) and good amount of improvement in the performance was observed as compared to the transverse ribs. Flow visualization studies through a duct roughened with inclined ribs (Taslim et al, 1996) revealed that, flow moving along the ribs (secondary flow) after striking the end wall of the duct, takes the shape of a recirculation vortex. This secondary flow vortex carries along cool fluid from locations away from the heated surface and forces it to strike the heated surface near the leading edge of the rib, thereby picking heat from it.…”

“…Han et al (1978) evaluated the effect of angle of inclination of a continuous rib on the heat transfer and friction characteristics and reported that angling of the rib from 90 o to 45 o resulted in steep fall in the friction factor with only 5% decrease in the average Stanton number, which makes angled ribs more suitable for optimum thermohydraulic performance. With the obvious benefit of having two leading edges per rib, Taslim et al (1996) Narinderpal Singh Deo, Subhash Chander, and J. S. Saini / American Journal of Heat and Mass Transfer (2016) Vol. 3 No.…”