The artificial neural network (ANN) methods are introduced (mainly for calculation of thermal and hydraulic coefficients) into a computer-aided design code of compact heat exchangers (CCHE). CCHE integrates the optimization, database, and process drawing into a software package. In the code, a strategy is developed for the optimization of compact heat exchangers (CHEs), which is a problem with changeable objective functions and constraints. However, the applicability and/or accuracy of all these methods are limited by the availability of reliable data sets of the heat transfer coefficients (j or Nu) and friction factors (f ) for different finned geometries. In fact, due to expenses and difficulties in experiments, only a limited number of experiments has been carried out for some kinds of heat transfer surfaces. The information, therefore, is usually given by means of correlations. It is well known, however, that the errors in the predicted results by means of correlations are much larger than the measurement errors, being mainly due to the data reduction represented by them. This implies doubts on the optimal solutions. Fortunately, a well-trained network is capable of correlating the data with errors of the same order as the uncertainty of the measurements. This is the main reason for the present introduction of the ANN method to correlate the discrete experimental data sets into continuous formulas. In this study, the ANN method is used to formulate the complex relationship between the thermal and hydraulic coefficients and the other parameters, including the geometry and process data. A specific case on the optimal analysis of a plate-fin heat exchanger (PFH) is presented to show how the trained ANNs can be used for optimal design of heat exchangers. In addition, a case is presented to illustrate how an inverse heat transfer problem is solved by the optimization methodology developed in the present code.
Numerical simulations coupled with laser Doppler velocimetry (LDV) experiments were carried out to investigate a slot jet issued into a cross flow, which is relevant in the film cooling of gas turbine combustors. The film-cooling fluid injection from slots or holes into a cross flow produces highly complicated flow fields. In this paper, the time-averaged Navier-Stokes equations were solved on a collocated body-fitted grid system with the shear stress transport k−ω, V2F k−ϵ, and stress-ω turbulence models. The fluid flow and turbulent Reynolds stress fields were compared to the LDV experiments for three jet angles, namely, 30, 60, and 90 deg, and the jet blowing ratio is ranging from 2 to 9. Good agreement was obtained. Therefore, the present solution procedure was also adopted to calculations of 15 and 40 deg jets. In addition, the temperature fields were computed with a simple eddy diffusivity model to obtain the film-cooling effectiveness, which, in turn, was used for evaluation of the various jet cross-flow arrangements. The results show that a recirculation bubble downstream of the jet exists for jet angles larger than 40 deg, but it vanishes when the angle is <30deg, which is in good accordance with the experiments. The blowing ratio has a large effect on the size of the recirculation bubble and, consequently, on the film cooling effectiveness. In addition, the influence of boundary conditions for the jet and cross flow are also addressed in the paper.
Impingement and forced convection are preferable methods for cooling gas turbine components. However, influences of various design parameters like crossflow and surface enlargements (like ribs) are not well understood. Thus there is a request for reliable and cost-effective computational prediction methods, due to the experimental difficulties. Such methods could be based on the numerical solution of the Reynolds-averaged Navier-Stokes equations, the energy equation and models for the turbulence field. This paper describes some recent advances and efforts to develop and validate computational methods for simulation of impingement and forced convection cooling in generic geometries of relevance in gas turbine cooling. Single unconfined round air jets, confined jets with crossflow, and three-dimensional ribbed ducts are considered. The numerical approach is based on the finite volume method and uses a co-located computational grid. The considered turbulence models are all the so-called low Reynolds number models. Our recent investigations show that linear and non-linear two-equation turbulence models can be used for impinging jet heat transfer predictions with reasonable success. However, the computational results also suggest that an application of a realizability constraint is necessary to avoid over-prediction of the stagnation point heat transfer coefficients. For situations with combined forced convection and impingement cooling it was revealed that as the crossflow is squeezed under the jet, the heat transfer coefficient is reduced. In addition, inline V-shaped 458 ribs pointing upstream performed superior compared to those pointing downstream and transverse ribs.
Experimental studies have revealed that both downstream and upstream pointing V-shaped ribs result in better heat transfer enhancement than transverse straight ribs of the same geometry. Secondary flows induced by the angled ribs are believed to be responsible for this higher heat transfer enhancement. Further investigations are needed to understand this. In the present study, the heat and fluid flow in V-shaped-ribbed ducts is numerically simulated by a multi-block 3D solver, which is based on solving the Navier-Stokes and energy equations in conjunction with a low-Reynolds number k-ε turbulence model. The Reynolds turbulent stresses are computed with an explicit algebraic stress model (EASM), while turbulent heat fluxes are calculated with a simple eddy diffusivity model (SED). Firstly, the simulation results of transverse straight ribs are validated against the experimental data, for both velocity and heat transfer coefficients. Then, the results of different rib angles (45° and 90°) and Reynolds number (15,000–30,000) are compared to determine the goodness of different rib orientations. Detailed velocity and thermal field results have been used to explain the effects of the inclined ribs and the mechanisms of heat transfer enhancement.
Numerical simulations coupled with LDV experiments were carried out to investigate a slot jet issued into a cross flow, which is relevant in the film cooling of gas turbine combustors. The film cooling fluid injection from slots or holes into a cross-flow produces highly complicated flow fields. In this paper, the time-averaged Navier-Stokes equations were solved on a collocated body-fitted grid system with the V2F turbulence model. The fluid flow and turbulent Reynolds stress fields were compared with the LDV experiments for three jet angles, namely, 30-deg, 60-deg, and 90-deg, and the jet blowing ratio is ranging from 2 to 9. Good agreement was obtained. Therefore, the present solution procedure was also adopted to calculations of 15-deg and 40-deg jets. In addition, the temperature fields, which were difficult to measure by experimental methods, were also computed with a simple eddy diffusivity model to obtain the film cooling effectiveness which was used for evaluation of the various jet-cross-flow arrangements. The results show that a recirculation bubble downstream the jet exists for jet angles larger than 40-deg, but it vanishes when the angle is less than 30-deg, which is in good accordance with the experiments. The blowing ratio has a large effect on the size of the recirculation bubble, and consequently on the film cooling effectiveness. In addition, the influence of boundary conditions for the jet and cross-flow are also addressed in the paper.
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