Conjugate Heat Transfer and Film Cooling of a Multi-Stage Cooling Scheme

Cooling techniques play a key role in improving efficiency and power output of modern gas turbines. The conjugate technique of film and impingement cooling schemes is considered in this study. The Multi-Stage Cooling Scheme (MSCS) involves coolant passing from inside to outside turbine blade through two stages. The first stage; the coolant passes through first hole to internal gap where the impinging jet cools the external layer of the blade. Finally, the coolant passes through the internal gap to the second hole which has specific designed geometry for external film cooling. The effect of design parameters, such as, offset distance between two-stage holes, gap height, and inclination angle of the first hole, on upstream conjugate heat transfer rate and downstream film cooling effectiveness performance are investigated computationally. An Inconel 617 alloy with variable properties is selected for the solid material. The conjugate heat transfer and film cooling characteristics of MSCS are analyzed across blowing ratios of Br = 1 and 2 for density ratio, 2. This study presents upstream wall temperature distributions due to conjugate heat transfer for different gap design parameters. The maximum film cooling effectiveness with upstream conjugate heat transfer is less than adiabatic film cooling effectiveness by 24–34%. However, the full coverage of cooling effectiveness in spanwise direction can be obtained using internal cooling with conjugate heat transfer, whereas adiabatic film cooling effectiveness has narrow distribution.

On Length Scales for the Radial Rotating Jet Pressure Exchange Mechanism

The crypto-steady rotating jet pressure exchange ejector is a novel concept in turbomachinery where two fluids, at different energy levels, come in direct contact with each other to transfer energy and momentum between them through non-steady interface pressure forces. The current paper seeks to provide an insight into the complex flow phenomena occurring inside the radial flow pressure exchange ejector. The primary mechanisms controlling the process are pressure exchange and mixing. This paper will seek to discriminate between energy transfer by each respective mechanism. The energy and momentum transfer in the near field is shown to be mainly due to the pressure exchange process, as the mixing layer does not develop substantially in this region. As the radius increases, the mixing layer tends to grow and the energy and momentum transfer is governed by the mixing process. As a consequence, the length scales of the pressure exchange zone are small, thus making the pressure exchange ejector more compact in size. The paper will delineate between the two length scales. If this new concept is shown to be viable for gas compression at sufficiently high pressure ratios, then, in refrigeration applications, it would enable environmentally benign refrigerants to replace the harmful chlorofluorocarbons (CFC) and reduce the effluence of greenhouse gases. Applications in many other areas, where conventional ejectors are currently used, are also possible.

Numerical Simulation of Immersion Quench Cooling Process: Part I

In this article, we describe a newly developed modeling procedure to simulate the immersion quench cooling process using the commercial code AVL-FIRE. The boiling phase change process, triggered by the dipping hot solid part into a subcooled liquid bath and the ensuing two-phase flow is handled using an Eulerian two-fluid method. Mass transfer effects are modeled based on different boiling modes such as film or nucleate boiling regime prevalent in the system. Separate computational domains constructed for the quenched solid part and the liquid (quenchant) domain are numerically coupled at the interface of the solid-liquid boundaries using the AVL-Code-Coupling-Interface (ACCI) feature. The advanced ACCI procedure allows the information pertaining to the phase change rates in the liquid domain to appear as cooling rates on the quenched solid boundaries. As a consequence, the code handles the multiphase flow dynamics in the liquid domain in conjunction with the temperature evolution in the solid region in a tightly coupled fashion. The methodology, implemented in the commercial code AVL-FIRE, is exercised in simulating the quenching of solid parts. In part I of the present research, phase change models are validated by simulating a work piece quenching process for which measurement data are available for various water temperature ranging from 20C to 80C. The computations provide a detailed description of the vapor and temperature fields in the liquid and solid domain at various time instants. In particular, the modifications arising in the liquid-vapor flow field in the near vicinity of the solid interface as a function of the boiling mode is well accommodated. The temperature history predicted by our model at different monitoring points, under different subcooling conditions, correlate very well with the experimental data wherever available. In part II, the model is further applied to real engine cylinder head quenching process and assessment is made for the cooling curves for various measuring points. Overall, the predictive capability of the new quenching model is well demonstrated.

Research on Heat Flux Distribution in Deep Grinding

This paper studies the heat transfer mechanism in deep grinding process, especially the heat flux to the workpiece. On the basis of triangle moving heat source, a quadratic curve heat flux model in the grinding zone was developed to determine the heat flux distribution and to estimate the surface temperature of workpiece. From the calculated theoretical expression of heat flux to the workpiece, the quadratic curve heat flux can be understood as the superposition of square law heat flux, triangular heat flux and uniform heat flux in the grinding zone. Then four heat flux models using the determined amount of heat flux were applied to estimate the workpiece surface temperatures which were compared with that measured by the embedded thermocouple. It has been found that the quadratic curve heat flux distribution seems to give the best match with measured and theoretical temperature, although square law heat flux model is good enough to predict the temperature.

Experimental Studies of Adiabatic Flow Boiling in Fractal-Like Branching Micro-Channels

Experimental results of adiabatic boiling of water flowing through a fractal-like branching microchannel network are presented and compared to numerical simulations for identical flow conditions. The fractal-like branching channel network had channel length and width ratios between adjacent branching levels of 0.7071, a total flow length of 18 mm, a channel height of 150 μm and a terminal channel width of 100 μm. The channels were DRIE etched into a silicon disk and pyrex was anodically bonded to the silicon to form the channel top and allowed visualization of the flow within the channels. The water flowed from the center of the disk where the inlet was laser cut through the silicon to the periphery of the disc. The flow rates ranged from 100 to 225 g/min and the inlet subcooling levels varied from 0.5 to 6 °C. Pressure drop across the channel as well as void fraction in each branching level were measured for each of the test conditions. The measured pressure drop ranged from 20 to 90 kPa, and the measured void fraction ranged from 0.3 to 0.9. The pressure drop results agree well with the numerical predictions. The measured void fraction results followed the same trends as the numerical results.

Design of Streamwise Stagger Within a Pin Fin Array to Minimize Pressure Loss

An experiment was conducted on the effects of streamwise stagger on heat transfer and pressure drop in a pin-fin array. The data were analyzed so as to highlight how stagger could be used to design a pin fin array for the lowest possible pressure loss. Design of arrays for low pressure loss is important in electronics cooling applications. They require large amounts of heat to be extracted from fixed areas, using a minimum of power to do so. This analysis found that the minimum friction factor occurred at a streamwise stagger of approximately 12% of the range between fully inline and fully staggered. By fixing the pin diameter, varying the stagger resulted in a 63% reduction in friction factor with only a 18% reduction in the Nusselt number, based on the array footprint. Additionally, it was found that for a fixed Nusselt number, the pin diameter could vary within a finite range, with decreasing diameters permitting arrays with more efficient degrees of stagger which continued to carry the required heating/cooling load.

Heat Flux Determination From Ultrasonic Pulse Measurements

Ultrasonic time of flight measurements have been used to estimate the interior temperature of propulsion systems remotely. All that is needed is acoustic access to the boundary in question and a suitable model for the heat transfer along the path of the pulse train. The interior temperature is then deduced from a change in the time of flight and the temperature dependent velocity factor, which is obtained for various materials as a calibration step. Because the acoustic pulse samples the entire temperature distribution, inverse data reduction routines have been shown to provide stable and accurate estimates of the unknown temperature boundary. However, this technique is even more interesting when applied to unknown heat flux boundaries. Normally, the estimation of heat fluxes is even more susceptible to uncertainty in the measurement compared to temperature estimates. However, ultrasonic sensors can be treated as extremely high-speed calorimeters where the heat flux is directly proportional to the measured signal. Through some simple one-dimensional analyses, this work will show that heat flux is a more natural and stable quantity to estimate from ultrasonic time of flight. We have also introduced an approach for data reduction that makes use of a composite velocity factor, which is easier to measure.

3D Simulations and Experimental Validation Using the Molecular Strain Function Model With Convective Constraint Release

The Molecular Strain Function model with Convective Constraint Release has been demonstrated by Wagner to fit elongational and shear viscosities, and First and Second Normal stress differences for a variety of polymer melts, when used with a Convective Constraint Release mechanism [J. Rheol. 45 (2001), 1387]. A modification to the CCR mechanism was shown to give more accurate representation of corner vortices in an abrupt contraction flow [JNNFM 135 (2006), 68] for both planar and axisymmetric contraction flows. It is highly desirable to assess the model against 3D flows. A primary advantage of 3D simulation in assessing a constitutive model is that, experimentally, it is very difficult to produce truly 2D data; the side walls of a finite die affect stress birefringence measurements (since this is a ‘line of sight’ cumulative measurement), and also induce significant 3D motion into the flow. The existing 2-dimensional code has been extended to fully 3-dimensional flows using 27-node ‘brick’ elements, and using a number of developments to deal with tracking and storage problems inherent in 3D time-integral solution. The 3D code is assessed against known 2-dimensional solutions to verify its accuracy; the constitutive model is then assessed against experimental data for a 4:1 contraction ratio die, which has finite width (5:3 depth to inlet height), inducing 3D effects. Stress birefringence, vortex size, and cross-sectional flow rate data at a number of flow rates are compared. The model is shown to give good accuracy against this flow.

Introducing Film Cooling Uniformity Coefficient (CUC)

A well performance film cooling implies for a high cooling effectives accompanied with a wide cooling coverage. During the past six decades, film cooling effectiveness has been well defined with a specific relation to quantify it. However, despite of numerous film cooling research, there is not an explicit method to quantify the uniformity of a coolant film spread over the hot surfaces. This work introduces a cooling uniformity coefficient (CUC) to evaluate how well a coolant film spreads over a surface being cooled. Four different cases are computationally studied. In the three cases, a single jet is injected into a hot cross flow with different jet exit shapes (i.e. square, spanwise rectangular, and streamwise rectangular). The fourth case is a novel combined triple jet (CTJ) introduced in our previous work. The cross sections of all the systems are equal to maintain the same coolant mass flow rate injection into the hot cross flow. The CUC’s of the different cases are compared with each other at two blowing ratios of 0.5 and 1.5. It is proposed that in addition to the film cooling effectiveness, the CUC is a necessary parameter to evaluate how well a coolant film is spread over a hot surface.

Integral Transform Solutions for Diffusion in Heterogeneous Media

The Generalized Integral Transform Technique is employed in the hybrid numerical-analytical solution of heat diffusion problems in heterogeneous media. The GITT is utilized to handle the associated eigenvalue problem with aribitrarily space variable coefficients, defining an eigenfunction expansion in terms of a Sturm-Liouville problem of known solution. The formal solution is first applied in solving an example of space variable thermophysical properties found in heat transfer analysis of functionally graded materials (FGM), validated by the exact solution obtained through classical integral transforms in the specific situation of exponentially varying coefficients. Then, it is challenged in handling a double-layered system with abrupt variation of properties, and critically compared against the exact solution obtained by the classical integral transform method with the adequate discontinuous multi-region eigenvalue problem. The convergence behavior of the proposed expansions is then critically inspected and numerical results are presented to demonstrate the applicability of the general approach.