Rib Spacing Effect on Heat Transfer in Rotating Two-Pass Ribbed Channel (AR=1:2)

Abstract

Rib turbulators are commonly used to enhance the heat transfer within internal cooling passages of advanced gas turbine blades. Many factors affect the thermal performance of a cooling channel with ribs. This study experimentally investigates the effect of rib spacing on the heat transfer enhancement, pressure penalty, and thus the overall thermal performance in both rotating and nonrotating rectangular, cooling channels. In the 1:2 rectangular channels, 45 deg angled ribs are placed on the leading and trailing surfaces. The pitch of the ribs varies, so rib pitchto-height (P=e) ratios of 10, 7.5, 5, and 3 are considered. Square ribs with a 1:59 mm 1:59 mm cross section are used for all rib spacing, so the height-to-hydraulic diameter (e=D h ) ratio remains constant at 0.094. With a constant rotational speed of 550 rpm and the Reynolds number ranging from 5000 to 40,000, the rotation number in turn varies from 0.2 to 0.02. Because the skewed turbulators induce secondary flow along the length of the rib, the very close rib spacing of P=e 3 has the best thermal performance in both rotating and nonrotating channels. This close spacing yields the greatest heat transfer enhancement, whereas the P=e 5 spacing has the greatest pressure penalty. In addition, the effect of rotation is more pronounced in the channel with the rib spacing of 3. As more ribs are added, the channel is approaching a smooth channel, and the strength of the rotation induced vortices increases. Nomenclature A smooth = area of the smooth copper plate A total = area of the smooth copper plate and the ribs AR = channel aspect ratio, W:H Bo x = local buoyancy parameter D h = channel hydraulic diameter e = rib height f = friction factor f 0 = fully developed friction factor in nonrotating, smooth pipe H = channel height h = regionally averaged heat transfer coefficient k = thermal conductivity of the coolant L = length of the rib-roughened portion of the test section Nu = regionally averaged Nusselt number Nu 0 = Nusselt number of the fully developed turbulent flow in nonrotating smooth pipe P = rib spacing P i = pressure at the inlet of the test section P o = pressure at the outlet of the test section Pr = Prandtl number of the coolant q 00 loss = heat loss through the wall q 00 w = heat flux at the wall Re = Reynolds number, VD h = Ro = rotation number, D h =V R x = local radius of rotation T b;x = local coolant bulk temperature T f;x = local film temperature (T w;x T b;x =2) T w;x = local wall temperature V = bulk velocity in streamwise direction W = channel width = rib angle of attack = angle of channel orientation with respect to the axis of rotation = x = local coolant-to-wall density ratio [T w;x T b;x =T f;x ] = thermal performance = viscosity of the coolant = density of the coolant = rotational speed

Heat Transfer and Friction in a Square Channel with Ribs and Grooves

“…The friction factor was calculated based on the measured pressure loss from [10], as shown in Eq. (5):…”

“…Reducing the distance between ribs can cause the separated flow to impinge the subsequent downstream rib without inducing flow reattachment to the surface. Liu et al [10] and Huh et al [11] studied the effect of extremely small rib spacing (P∕e 3 and 2.5) on heat transfer by using a 45 deg angled rib configuration in rectangular channels. Another study reported that the 45 deg angled rib configuration was favorable for triangular-shaped cooling channels near the leading edge of a gas turbine blade [12].…”

Enhanced Internal Heat Transfer on the Tip-Wall in a Rectangular Two-Pass Channel (AR = 1:2) by Pin-Fin Arrays

“…Taking the advantage of impinging and turning effects, the tip can be cooled to a certain extent. During recent years, several studies have concerned turbulent heat transfer in various two-pass channels [7][8][9][10][11][12][13][14][15][16][17][18]. For example, Ekkad et al [7] measured the detailed heat transfer distributions inside straight and tapered two-pass channels with and without rib turbulators.…”

“…It was found that the tapered channel with ribs provided 1.5-2.0 times higher Nusselt number ratios over the tapered smooth channel in the first pass while in the after-turn region of the second pass the ribbed and smooth channels provided similar Nusselt number ratios. Fu et al [13,14] and Liu et al [15] reported heat transfer coefficients and friction factors in two-pass rectangular channels with rib turbulators placed on the leading and trailing surfaces. Five kinds of ribs were considered: 45 angled, V-shaped, discrete 45 angled, discrete V-shaped, and crossed V-shaped.…”

Numerical predictions of augmented heat transfer of an internal blade tip-wall by hemispherical dimples

“…Besides, the secondary flow characteristics were correlated with the wall heat transfer enhancement for smooth and ribbed wall two-pass square channels. Al-hadhrami and Han [8], Fu et al [9], and Liu et al [10] and reported heat transfer coefficients and friction factors in two-pass rectangular channels with rib turbulators placed on the leading and trailing surfaces. Five kinds of ribs were considered: 45°angled, V-shaped, discrete 45°angled, discrete V-shaped and crossed V-shaped.…”