Numerical Investigation of Heat Transfer Coefficient in Ribbed Rectangular Duct with Various Shaped Ribs and Different Coolants

Abstract

Abstract — In the present study, numerical studies are carried out to investigate the heat transfer in rectangular duct roughened by square and trapezoidal shaped ribs on one wall using different fluids. The coolant fluids such as air, steam, air/mist and steam/mist were investigated. The computational results show that the shear stress transport (SST) turbulence model is selected by comparing the predictions of different turbulence models with experimental results. The heat transfer coefficients enhance in ribbed channel at injection small amount of mist. The heat transfer coefficients of air/mist, steam/mist increase by 14% and 104% than that of air, respectively in square shaped ribs. However the heat transfer coefficient of air, air/mist, steam and steam/mist increase by 9%, 16%, 68% and 118%, respectively for trapezoidal shaped ribs compared with air in square shaped ribs. Keywords - Heat transfer; Rib roughness; mist I. I NTRODUCTION Periodic ribs are frequently employed to enhance the heat transfer process in various cooling passages such as turbine blades, guide vanes and combustor walls. Heat transfer augmentation inside cooling channels is achieved by using repeated ribs as turbulence promoters. The periodic ribs break the laminar sub-layer and create local wall turbulence due to flow separation and reattachment between the ribs, greatly enhancing the heat transfer. Several researchers have studied the heat transfer characteristics in straight channels with various shaped ribs using air as coolant flow. Chandra et al. [1], Han et al. [2], Lanjewar et al. [3], Srinath et al. [4], Salameh and Sunden [5], Tanda [6] and Wang and Sunden [7] studied experimentally the effect of ribs configuration and angled ribs on heat transfer and friction. While by numerical predictions of the flow and heat transfer in rib-roughened passages have been conducted previously by several investigators: Kashmiri et al. [8] investigated the rib pitch effect on heat transfer. Taslim and Liu [9], and Haasenritter et al. [10] performed both numerical and experimental analyses on roughened square channel with sharp and round profile ribs using k- turbulence emodel. They found good agreement between modeling and experimental. Chaube et al. [11] obtained a good agreement of heat transfer predicted with experimental data for roughness plate using SST k-ω. Lu and Jiang [12] have performed both the numerical analysis and experimental study to investigate the heat transfer and fluid flow behavior in rectangular channel using SST k-ω and RNG k-e turbulence models. They have concluded that the SST k-ω turbulence model was more suitable for the convection heat transfer in such channels. Wang et al. [13] examined the capabilities of different turbulence models in predicting heat transfer and flow serpentine cooling channel. They compared performance of air and steam as coolant flow. They showed the better model was SSG turbulence model and the steam heat transfer efficiency is higher than that of air. Shui et al. [14] compared of k-e model, SST model and SSG model in prediction of heat and fluid flow of square ribbed channel. The simulation matched well with experimental results using SSG turbulence model and the steam is proper coolant than air. Moreover, many numerical studies have been published regarding comparison analysis of air and steam as a coolant such as Albeirutty et al. [15], Najjar et al. [16] and Sanjay et al. [17]. These studies showed that the closed loop steam cooling offers the highest plant efficiency. There have been experimental and numerical studies of tubes and flat plate cooling with air/mist including those of Sikalo et al. [18], Oisin et al. [19], Novak et al. [20], Kumari et al. [21], Shokouhmand and Ghaffari [22] and Pakhomov and Terekhov [23]. These studies concluded that the heat transfer coefficient can be increased with introduction of a fine water mist. However the experimental and numerical validation of heat transfer results of mist/steam cooling in heated horizontal tube introduced by Gou et al. [24] and Dhanasekaran and Wang