Review: The flow and heat transfer investigation inside tapered and straight two pass channels with rib turbulators

This paper is a review of the number of experimental and CFD experiments performed with rib turbulators about the heat transfer and the at two pass channel. In respect to achieve higher thermal efficiency of gas turbines, efforts are made to raise the inlet temperature. The secondary fluxes resulting from the rib and U-shaped curvature play an important role enhancing heat transfer in the two pass ribbed channels. The ribs on the internal surface of cooling passage will increase the strength of heat transfer. This paper deals with the effect of tapered and straight two-pass channels with the influence of the variable rib cross-section on flow and heat transfer enhancement

Effect of Discrete Ribs on Heat Transfer and Friction Inside Narrow Rectangular Cross Section Cooling Passage of Gas Turbine Blade

The performance of a gas turbine during the service life can be enhanced by cooling the turbine blades efficiently. The objective of this study is to achieve high thermohydraulic performance (THP) inside a cooling passage of a turbine blade having aspect ratio (AR) 1:5 by using discrete W and V-shaped ribs. Hydraulic diameter (Dh) of the cooling passage is 50 mm. Ribs are positioned facing downstream with angle-of-attack (α) of 30° and 45° for discrete W-ribs and discerte V-ribs respectively. The rib profiles with rib height to hydraulic diameter ratio (e/Dh) or blockage ratio 0.06 and pitch (P) 36 mm are tested for Reynolds number (Re) range 30000-75000. Analysis reveals that, area averaged Nusselt numbers of the rib profiles are comparable, with maximum difference of 6% at Re 30000, which is within the limits of uncertainty. Variation of local heat transfer coefficients along the stream exhibited a saw tooth profile, with discrete W-ribs exhibiting higher variations. Along spanwise direction, discrete V-ribs showed larger variations. Maximum variation in local heat transfer coefficients is estimated to be 25%. For experimented Re range, friction loss for discrete W-ribs is higher than discrete-V ribs. Rib profiles exhibited superior heat transfer capabilities. The best Nu/Nuo achieved for discrete Vribs is 3.4 and discrete W-ribs is 3.6. In view of superior heat transfer capabilities, ribs can be deployed in cooling passages near the leading edge, where the temperatures are very high. The best THPo achieved is 3.2 for discrete V-ribs and 3 for discrete W-ribs at Re 30000. The ribs can also enhance the power-toweight ratio as they can produce high thermohydraulic performances for low blockage ratios.

Effect of a Cooling Unit on High-speed Motorized Spindle Temperature With a Scaling Factor

Forced cooling, as an efficient way of heat dissipation, significantly affects the spindle temperature. Although a full cooling passage was factored into the finite element analyses by some scholars, often only the model for the front or rear half of a spindle is need for the purpose of the thermal evaluation simplification and data overhead reduction in engineering applications. So far, how the coolant passage affects the heat dissipation of the front or rear half of a spindle has not been well characterized. This paper devotes to constructing a scaling factor to represent the coolant unit effect on the thermal growth of spindles. The experiments about the effect of coolant units on spindle temperature were first implemented, and then the qualitative conclusions were got with various coolant parameter settings. To further quantify these influences, the regressive analysis was carried out. As a result, the peak temperature area was found and the scaling factors were proposed to describe the effect of the cooling system on the front or rear half of spindle temperature. In this process, the thermal equivalent convection for coolant passage was modeled based on the thermal resistance theory. In the meantime, we planned a novel thermal network of a motored spindle for contrast and validation, in which the cooling mechanism was integrated, and the structural constraints were considered by the aid of the proposed scaling factors. The result is indicative of a better agreement with real values when employing the proposed model.

Investigating Heat Transfer in a Straight Cooling Passage Using Transient Infrared Temperature Data and URANS Conjugate Heat Transfer Analysis

Experimental work measuring heat transfer due to internal convection on a smooth straight passage is recreated using unsteady Reynolds averaged Navier-Stokes conjugate heat transfer simulations. The experimental work utilizes 1-dimensional and 3-dimensional conduction models to determine internal heat transfer rates from external surface temperature measurements collected with an infrared camera. The numerical simulations recreated these experiments to verify the conduction model and investigate the differences between the k-ω shear stress transport turbulence model, Reynolds stress turbulence model, and the k-ε turbulence model. It is found that the conduction model can accurately predict the heat transfer in the passage within an average error of 6% but with reduced spatial accuracy. The lower spatial accuracy can be accounted for by utilizing both the conduction model to predict the magnitude of the heat transfer and the numerical simulations to capture the spatial distribution. No one turbulence model was found to provide consistently superior heat transfer predictions, but rather each model excelled in some scenarios and underperformed in others. Overall, the k-ε model was found to best match the experimental heat transfer calculations with an average error of 5.9% of the total heat transfer, and it takes a more conservative approach as it can over predict the external surface temperatures by approximately 0.4 K. The end goal of this study is to develop a way to derive heat-flux data from infrared measurements on a range of geometries. A simple and well-understood geometry is investigated here to provide a firm foundation for future work.

Fully Coupled Large Eddy Simulation of Conjugate Heat Transfer in a Ribbed Channel with a 0.1 Blockage Ratio

Large eddy simulations are performed to analyze the conjugate heat transfer of turbulent flow in a ribbed channel with a heat-conducting solid wall. An immersed boundary method (IBM) is used to determine the effect of heat transfer in the solid region on that in the fluid region in a unitary computational domain. To satisfy the continuity of the heat flux at the solid–fluid interface, effective conductivity is introduced. By applying the IBM, it is possible to fully couple the convection on the fluid side and the conduction inside the solid and use a dynamic subgrid scale model in a Cartesian grid. The blockage ratio (e/H) is set at 0.1, which is typical for gas turbine blades. Through conjugate heat transfer analysis, it is confirmed that the heat transfer peak in front of the rib occurs because of the impinging of the reattached flow and not the influence of the thermal boundary condition. When the rib turbulator acts as a fin, its efficiency and effectiveness are predicted to be 98.9% and 8.32, respectively. When considering conjugate heat transfer, the total heat transfer rate is reduced by 3% compared with that of the isothermal wall. The typical Biot number at the internal cooling passage of a gas turbine is <0.1, and the use of the rib height as the characteristic length better represents the heat transfer of the rib.

Heat Transfer in Rotating, Trailing-Edge, Converging Channels With Smooth Walls and Pin-Fins

The heat transfer and pressure drop characteristics of a rotating cooling channel that has an angled trapezoidal cross section and converges from the hub to the tip in both the streamwise and spanwise directions are experimentally investigated. The channel is oriented 120 deg with respect to the direction of rotation to model the geometry of an internal, trailing-edge cooling passage. Both the leading and trailing sides of the channel are divided into three and six regions in the spanwise and streamwise directions, respectively. The copper plate method is used to obtain regionally averaged heat transfer coefficients. The pressure drop is measured using pressure taps placed at the inlet and outlet of the channel. Experiments were conducted with the inlet Reynolds number ranging from 10,000 to 40,000. The rotational speed varies from 0 rpm to 300 rpm, resulting in the highest rotation number of 0.21. The effects of full pin-fins on the heat transfer and pressure drop characteristics are obtained and compared to the smooth surface converging channel results. The impact of the convergence, which causes variations of flow and geometric parameters through the passage, such as aspect ratio, Reynolds number, and rotation number, on the heat transfer coefficients and pressure drop are addressed. Results show that due to the 120 deg channel orientation, the rotation has a positive impact on the leading and trailing surface heat transfer. Furthermore, the convergence decreases the aspect ratio while increasing the Reynolds number. The convergence significantly enhances heat transfer on both the leading and trailing surfaces along the streamwise and spanwise directions. The convergence also reduces the rotation effect in the streamwise direction for a given mass flow rate.

Heat Transfer in Rotating, Trailing Edge, Converging Channels With Partial Length Pin-Fins

In the current study, the heat transfer and pressure drop characteristics of a rotating, partial pin-finned, cooling channel that has a trapezoidal cross section and converges from the hub to tip in both the streamwise and spanwise directions are experimentally investigated. To model the geometry of an internal trailing edge cooling passage, the channel is oriented with respect to the direction of rotation (β = 120 deg). Isolated copper plates are used to obtain regionally averaged heat transfer coefficients on the leading and trailing surfaces. Pressure drop is measured using pressure taps placed at the inlet and outlet of the channel. Utilizing Dp = 5 mm diameter pins, a staggered array is created. For this array, the streamwise pin-spacing, Sy/Dp = 2.1, was kept constant; however, the spanwise pin-spacing, Sx/Dp, was varied from the hub to tip between 3 and 2.6 due to the channel convergence. Experiments were conducted for two partial pin-fin sets having pin length-to-diameter ratios of Sz/Dp = 0.4 and 0.2. The rotation number was varied from 0 to 0.21 by ranging the inlet Reynolds number from 10,000 to 40,000 and rotation speed from 0 to 300 rpm. A significant decrease in pressure loss and a slight reduction in heat transfer enhancement are observed with the use of partial pin-fins compared with the previously reported full pin-fin converging channel study. This provides better thermal performances of the partial pin-fin arrays compared with the full pin-fin array, in the converging channels.

Fully Coupled Large Eddy Simulation-Conjugate Heat Transfer Analysis of a Ribbed Cooling Passage Using the Immersed Boundary Method

The study focuses on evaluating fully coupled conjugate heat transfer (CHT) simulation in a ribbed cooling passage with a fully developed flow assumption using large eddy simulation (LES) with the immersed boundary method (IBM-LES-CHT). The IBM-LES and the IBM-CHT frameworks are validated by simulating purely convective heat transfer in the ribbed duct, and a laminar boundary layer flow over a 2D flat plate with heat conduction, respectively. For the main conjugate simulations, a ribbed duct geometry with a blockage ratio of 0.3 is simulated at a bulk Reynolds number of 10,000 with a conjugate boundary condition applied to the rib surface. The nominal Biot number is kept at 1, which is similar to the comparative experiment. It is shown that the time scale disparity between turbulent fluid flow and heat conduction in solid can be overcome by using an artificially high solid thermal diffusivity. While the diffusivity impacts the instantaneous fluctuations in temperature and heat transfer, it has an insignificant effect on the predicted Nusselt number. Comparison between IBM-LES-CHT and iso-flux heat transfer simulations shows that the iso-flux case predicts higher local Nusselt numbers at the back face of the rib. Furthermore, the local Nusselt number augmentation ratio (EF) predicted by IBM-LES-CHT is compared with experiment and another LES conjugate simulation. The present LES calculations predict higher EFs on the leading face of the rib and show a different trend at the trailing face when CHT is activated.