Heat transfer and exergy analysis of flow boiling evaporation in u-bend channel

This study presents, a numerical method used to evaluate the exergy analysis of flow boiling evaporation of R134a in a U-bend channel using entropy generation criterion which is concerned with the degradation of exergy during the process due to irreversibilies (entropy generation) contributed by heat transfer and pressure drop. The simulations were conducted with the heat flux of 15 kW/m2, mass fluxes of 200-600 kg/m2s of R134a at the saturation temperature of 15 °C. Three(3) different geometries sizes of U-bend channel’s diameter 6, 8 and 10 mm with the bend radius of 10.2 mm were utilized. The Volume of Fluid (VOF) multiphase flow formulation was used in Ansys Fluent. The results show that the entropy generation increases with increase in mass fluxes due to irreversibilies contributed by the heat transfer coefficient and pressure drop as mass fluxes increase. Based on the size of the U-bend channel, the entropy generation was found to increase as the diameter of the channel increases. The numerical results were compared with the data in the open literature and there was a good agreement.

Optimization of an Additively Manufactured U-Bend Channel Using A Surrogate-Based Bayesian Method

CFD-based design optimization of turbulent flow scenarios is usually computationally expensive due to requirement of high-fidelity simulations. Previous studies prove that one way to reduce computational resource usage is to employ Machine Learning/Surrogate Modeling approaches for intelligent sampling of data points in the design space and is also an active area of research, but lacks enough experimental validation. Such a method has been used to optimize the shape of a U-bend channel for the minimization of pressure drop. U-bends are an integral part of serpentine cooling channels inside gas turbine blades but also contribute to total pressure drop by more than 20%. Reducing this pressure loss can help in more efficient cooling at low pumping power. A ‘U-bend’ or 180-degree bend shape has been used from literature, and a 16-dimensional design space has been created using parametrized spline curves, which creates a variety of shapes inside a given bounding box. A Latin Hypercube Sampling (LHS) was carried out for populating the initial design space with output data from the CFD simulation. After training a surrogate model on this data set, Bayesian updates were used to search for an optimum using an exploration vs exploitation approach. The resulting optimum shape showed that pressure drop was lowered by almost 30%, when compared to the baseline. The aim of this study is to experimentally validate this method using 3D printed models of the baseline and optimum channels respectively. Pressure taps placed across stream-wise locations on these channels helped to create a pressure profile for turbulent flow at a Reynolds number of 17000, for comparison to CFD results.

Study On Flow and Heat Transfer Characteristics of a New-Proposed Alternating Elliptical U-Channel in the Mid-Chord Region of Gas Turbine Blade

This paper proposed an alternating elliptical U-bend cooling channel which can be applied in the mid-chord region of gas turbine blade and manufactured by precision casting, based on the optimal flow field structure deduced from the Field Synergy Principle, and investigated the flow and heat transfer characteristics in this alternating elliptical U-bend cooling channel thoroughly. Numerical simulations were performed by using 3D steady solver of Reynolds-averaged Navier-Stokes equations (RANS) with the standard k-e turbulence model. The influence of alternating of cross section on heat transfer and pressure drop of the channel was studied by comparing with the smooth elliptical U-bend channel. On this basis, the effect of aspect ratio (length ratio of the major axis to the minor axis) and alternating angle were further investigated. The results showed that, in the first pass of the alternating elliptical U-bend channel, for different Re, four or eight longitudinal vortices were generated. In the second pass, the alternating elliptical channel restrained the flow separation to a certain extent and a double-vortex structure was formed. The average Nusselt number of the alternating elliptical U-bend channel was significantly higher than that of the straight channel, but the pressure loss only increased slightly. With the increase of aspect ratio, the thermal performance of the channel increased, and when the alternating angle is between 40° and 90°, the thermal performance nearly kept constant and also the best.

Numerical Simulation of Multiphase Flow on Temperature Effect for Proton Exchange Membrane Fuel Cell

There is a significant effect for the performance of proton exchange membrane fuel cell with the liquid water generated in cathode channel during the operation process In this paper, based on the numerical simulation of three-dimensional and VOF model of multiphase flow, according to the different flow channel design and the change of different inlet temperature, the transport phenomena of multiphase flow in PEMFC is discussed at temperature effect. In U-shaped cathode channel with bump scale (h/H=1/4), a long water film is assumed to cover the surface of the gas diffusion layer at the entrance. Under the simulated oxygen flow conditions of inlet 200 Reynolds number, 4.4 Weber number and inlet temperature 333K, the water film heated is not obviously affected by oxygen flow from inlet channel to bend channel. Subsequently, the shape of water film is elongated and broken from outflow of bend channel to outlet channel. The computational results are obtained that the residual broken water film can be existed in the outlet channel. The flow field temperature can affect the residual flow rate of water film in the channel. The residual rate of water film in the hot flow field is lower than that in the cold flow field.

Heat Transfer Characteristics of Blade Internal Tip Surface by Arrays of Delta-Winglet Vortex Generator Pairs

This paper numerically investigated the effect of the arrays of the delta-winglet vortex generators (DWVGs) pairs on the flow field and heat transfer characteristics of gas turbine blade tip internal surface. Six different arrangements including three inclinations (30°, 45°, 60°) and two aspect ratios of DWVGs are calculated at Reynolds numbers ranging from 6000 to 14,000. The internal cooling passage of gas turbines are simplified as two-pass U bend channel and the U bend channel without any tabulators are considered as Baseline. The detailed flow structure, the evolution of vortices and heat transfer performance over the tip internal surface are presented. The results show that the arrays of DWVGs placed on the tip internal surface have great influence on the tip flow and heat transfer. Small-scale vortices are induced by the DWVGs, which have negligible impact on the main flow. Due to the nature of 180-deg turn, the impingement-like flow contributes the highest heat transfer performance. But too many DWVGs placed on the attachment region will weaken the energy of main flow and therefore reduce the local heat transfer. Besides, the blocked DWVGs (BVG) will enhance the heat transfer at the center line, and the guided DWVGs (GVG) will extend the low-energy flow cluster and thus weakening the heat transfer intensity. The results of this study are useful for understanding the mechanism of heat transfer characteristics in a realistic gas turbine blade by using the arrays of DWVGs.

Effect of Ribs on Heat Transfer and Pressure Drop in a U-Bend Channel With Double-Layer, Dome-Shaped Turning Vanes

In this paper, the combined effects of ribs and double-layer, dome-shaped turning vanes on heat transfer and pressure drop are investigated in an idealized U-bend channel. Five kinds of ribs including transverse ribs, 45° ribs, 135° ribs, V-shaped ribs, and reverse V-shaped ribs combined with one kind of double-layer, dome-shaped turning vanes are applied. Baseline results are compared with the above composite cooling structures. Numerical simulations are performed by solving 3D, steady Reynolds-averaged Navier-Stokes (RANS) equations with k-ω turbulence model. The channel aspect ratio is 1:2 and its hydraulic diameter is 93.13 mm, respectively. Based on the cooling air inlet velocity and the channel inlet hydraulic diameter, the inlet Reynolds numbers are ranging from 100,000 to 440,000. The detailed three-dimensional fluid flow, pressure and heat transfer distributions are presented. Moreover, the thermal performances of the U-bend channel are also evaluated and compared with different cases. The results revealed that combined with the double-layer, dome-shaped turning vanes, the transverse ribs case has the best thermal performance at the tip wall, and the reverse V-shaped ribs case is the best for the leading wall. The pressure drop of the channel with double-layer, dome-shaped turning vanes without any rib turbulator is the lowest, and that of the channel with inclined ribs is significantly higher than that of the channel with transverse ribs. The superposition of the secondary flow induced by the ribs and the Dean vortex induced by the 180° sharp turn has a marked impact on the flow and heat transfer in the channel. In the double-layer, dome-shaped turning vanes channel, the mass flow distribution of the coolant also affects the heat transfer on the tip wall of the channel, and the ribs can adjust the mass flow distribution. The helical vortex superposed by the mainstream flow and the secondary flow induced by the ribs represents typical flow phenomenon in ribbed channels. The flow and development of the helical vortex are the main factors affecting the heat transfer on the leading/trailing walls.