Heat transfer distribution in rectangular ducts with V-shaped ribs

Abstract

Heat transfer distributions are presented for a rectangular duct with two opposite wide walls arranged with V-shaped ribs pointing upstream or downstream relative to the main¯ow direction. The rectangular duct has an aspect ratio of 1/8. The parallel V-shaped circular ribs are arranged staggered on the two wide walls. The rib height-to-hydraulic diameter ratio is 0.06, with an attack angle of 60°. The pitch-to-height ratio equals 10. The tested Reynolds numbers range from 1000 to 6000. The test surface is sprayed with black paint and then liquid crystal, and a steady state method is adopted to obtain the temperature distribution between adjacent ribs. The secondary¯ow caused by the angled ribs creates different spanwise variation of the heat transfer coef®cient on the rib-roughened wall for different V-rib orientations. Interaction between heat transfer and secondary¯ow is analyzed. In the streamwise direction, the temperature distribution shows a sawtooth behavior between a pair of adjacent ribs. Local Nusselt numbers are presented between a pair of adjacent ribs, and based on these the average Nusselt numbers are calculated to investigate the augmentation of heat transfer by the presence of the V-shaped ribs. IntroductionPeriodic ribs are frequently employed in heat exchangers to enhance the heat transfer process. The heat transfer distribution and enhancement caused by parallel ribs has been investigated [1]. V-shaped ribs originated from the concept that angled ribs can act as longitudinal vortex generator and provide heat transfer augmentation. It is then assumed that V-shaped ribs can principally double the high heat transfer region and provide even higher heat transfer coef®cients (see, e.g., [2]). Because the¯ow ®eld is disturbed by the presence of the ribs and the orientation of the V-shaped ribs will induce different types of secondarȳ ow, and the local heat transfer distributions are also different.Han et al. used thermocouples to investigate the heat transfer of a square duct with 45 and 60°V-shaped ribs, and the ribs could point in the main¯ow direction or in the opposite direction [3]. The ribbed walls were in-line and the Reynolds number covered the range of 15 000± 90 000. The V-shaped ribs directed opposite to the main ow provided the highest heat transfer enhancement. Taslim et al. employed liquid crystal thermography to study the heat transfer enhancement caused by V-shaped ribs on two opposite walls of a square duct [2]. The 45°angled ribs were arranged with a ®xed pitch-to-height ratio of 10. In the whole Reynolds number range of 5000±30 000, for the blockage ratio (e/D h ) of 0.083, it was revealed that V-shaped ribs pointing downstream produced the highest heat transfer enhancement. These results contradict those of [3]. In another investigation, Ekkad and Han measured the heat transfer of a two-pass square duct with 60°V-ribs using liquid crystal thermography. The Reynolds numbers were from 6000 to 60 000 [4]. The Nusselt number ratio distributions for the ®rst pass with V-ribs pointing in...

Nusselt number and friction factor correlations for solar air heater duct with broken V-down ribs combined with staggered rib roughness

“…[1][2][3][4][5] The secondary flow originated by rib inclination interacts with main flow ensuing higher turbulence and heat dissipation. Taslim et al 4 observed that V-rib produces twin secondary flow cells progressing along the V limbs result in notable enhancement in heat transfer over inclined rib.…”

“…The effect of V-rib on heat and fluid flow has been experimentally investigated by several investigators for different geometrical parameters of roughness and duct aspect ratios. [4][5][6][7][8][9][10][11][12][13][14] Many investigators 4,5,13 reported that V-rib pointing in the flow direction attains higher rates of heat flow than apex facing upstream for the range of Reynolds number normally used in solar air heaters. Tanda 15 explored that the thermo-hydraulic performance of duct flow could be considerably improved by discrete roughness elements.…”

Heat transfer and friction factor correlations for a solar air heater duct roughened artificially with V-ribs with symmetrical gaps

“…Various other studies show that the use of V-ribs with different orientation and positions enhances the heat transfer and boost up the performance of a solar air heater as well as other applications. (Han and Zhang, [10]; Lau et al, [11,12]; Han et al, [13,14]; Taslim and Spring, [15]; Taslim et al, [16]; Wang and Sunden [17]; Wright et al, [18]; Olsson and Sunden, [19]; Gao and Sunden, [20,21]; Jia et al, [22,23].…”

Numerical investigation of fluid flow structure and heat transfer in a passage with continuous and truncated V-shaped ribs

“…It was found that the 45-deg angled V-shaped ribs produced the higher heat transfer performance compared with other rib configurations. Gao and Sunden (2001) measured the heat transfer coefficients between adjacent pairs of 60-deg V-shaped ribs. The results showed that the local Nusselt number around the middle point between a pair of adjacent ribs was higher than that near the ribs in the main flow direction.…”