Local Heat/Mass Transfer Measurements in a Rectangular Duct With Discrete Ribs

Abstract

The influence of the arrangement and the length of discrete ribs on heat/mass transfer and friction loss is investigated. The mass transfer experiments are conducted to obtain detailed local heat/mass transfer coefficients on the duct wall. The aspect ratio (width/height) of the duct is 2.04 and the rib height is one tenth of the duct height, such that the ratio of rib height to hydraulic diameter is 0.0743. The ratio of rib-to-rib distance to rib height is 10. The discrete ribs are made by dividing continuous ribs into 2, 3 and 5 pieces and attached periodically to the top and bottom surfaces of the duct with a parallel orientation. After examining the effects of rib angle of attack (α) for continuous ribs, the combined effects of the rib angle and the length of discrete ribs on heat/mass transfer on the duct wall are investigated for α = 90° and 45°. As the number of broken pieces of a rib increases, the more disturbed flows affect greatly heat/mass transfer and increase the uniformity of heat/mass transfer distributions. For α = 90°, the heat/mass transfer enhancement with the discrete ribs is remarkable, so that the discrete ribs augment up to 27% of the average heat/mass transfer coefficients compared with the transverse continuous rib. However, the heat/mass transfer performances of the discrete ribs are slightly higher than that of the transverse continuous rib due to the accompanied high friction loss penalty. For α = 45°, the average heat/mass transfer coefficients are decreased slightly with the discrete ribs, and the heat/mass transfer performances of the angled discrete ribs are also decreased even though the friction losses are lower.

Experimental and numerical study on heat transfer enhancement in a channel with Z-shaped baffles

“…Cho et al [24] investigated the experiment of thermal characteristics in a rectangular duct with 45°and 90°discrete baffles, and they found that the most heat/mass transfer performances at about 1.2. Bhagoria et al [25] indicated that the Nu and f in a channel with wedge baffles at e/D ratios of 0.015-0.033 and baffle wedge angles of 8-15°increased by 2.4 and 5.3 times over smooth duct.…”

The effect of different inlet conditions of air in a rectangular channel on convection heat transfer: Turbulence flow

“…Methods of the PHE have been extensively explored with the attempts to overcome the thermal problems subject to high heat flux [4]. A variety of surface ribs [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21], perforated baffle [22][23][24][25][26][27], pin-fins [28][29][30][31][32], Snail, propeller, twisted and winglet-type vortex generators [33,34,30,[35][36][37][38][39][40][41][42][43][44][45][46] have been devised to enhance heat transfer rates in ducts. However, it is shown in the literature review that detail exploration of turned flow in the channel needs more extensive study.…”

“…A silicone rubber heater is fixed to the outer surface of the bottom of the test section to provide the uniform heat flux. The heat flux was set by adjusting the electrical voltage with the help of AC variable electric power supply (De-Lorenzo, with accuracy of ±1-1.5% for voltage (V) and current (I)), and the constant heat flux was allowed to continue till the steady state is attained (8). The temperature of the heated surface is measured by 11 calibrated and electrically insulated 0.5 mm diameter T-type thermocouples fixed at selected locations in the test section (9).…”

A review on methodology of artificial roughness used in duct of solar air heaters

“…Hu and Shen [19] investigated the effect of inclined discrete ribs with and without groove and reported performance improvement for discrete arrangement without groove. Cho et al [20] examined the effect of angle of attack and number of discrete ribs in rectangular duct and reported that gap region between discrete ribs accelerates the flow and results an increase in local heat transfer coefficient. Chyu et al [21,22] reported local heat transfer measurements on ribs of hemispherical and teardrop shapes by using a transit liquid crystal technique and obtained 2.5 times greater heat transfer enhancement and air pressure penalty is half the values produced by conventional rib turbulator.…”