Heat transfer characteristics within an array of impinging jets. Effects of crossflow temperature relative to jet temperature

Spanwise average heat fluxes, resolved in the streamwise direction to one stream-wise hole spacing were measured for two-dimensional arrays of circular air jets impinging on a heat transfer surface parallel to the jet orifice plate. The jet flow, after impingement, was constrained to exit in a single direction along the channel formed by the jet orifice plate and heat transfer surface. The crossflow originated from the jets following impingement and an initial crossflow was present that approached the array through an upstream extension of the channel. The regional average heat fluxes are considered as a function of parameters associated with corresponding individual spanwise rows within the array. A linear superposition model was employed to formulate appropriate governing parameters for the individual row domain. The effects of flow history upstream of an individual row domain are also considered. The results are formulated in terms of individual spanwise row parameters. A corresponding set of streamwise resolved heat transfer characteristics formulated in terms of flow and geometric parameters characterizing the overall arrays is described

Jet array impingement flow distributions and heat transfer characteristics: Effects of initial crossflow and nonuniform array geometry

Flow distributions and heat transfer characteristics for two-dimensional arrays of circular air jets impinging on a surface parallel to the jet orifice plate were determined. The configurations considered were intended to model those of interest in current and contemplated gas turbine airfoil midchord cooling applications. The geometry of the airfoil applications considered dictates that all of the jet flow, after impingement, exit in the chordwise (i.e., streamwise) direction toward the trailing edge. Experimental results for the effect of an initial crossflow on both flow distributions and heat transfer characteristics for a number of the prior uniform array geometries. The effects of nonuniform array geometries on flow distributions and heat transfer characteristics for noninitial crossflow configurations are discussed

Forced convection heat transfer to air/water vapor mixtures

Heat transfer coefficients were measured using both dry and humid air in the same forced convection cooling scheme and were compared using appropriate nondimensional parameters (Nusselt, Prandtl and Reynolds numbers). A forced convection scheme with a complex flow field, two dimensional arrays of circular jets with crossflow, was utilized with humidity ratios (mass ratio of water vapor to air) up to 0.23. The dynamic viscosity, thermal conductivity and specific heat of air, steam and air/steam mixtures are examined. Methods for determining gaseous mixture properties from the properties of their pure components are reviewed as well as methods for determining these properties with good confidence. The need for more experimentally determined property data for humid air is discussed. It is concluded that dimensionless forms of forced convection heat transfer data and empirical correlations based on measurements with dry air may be applied to conditions involving humid air with the same confidence as for the dry air case itself, provided that the thermophysical properties of the humid air mixtures are known with the same confidence as their dry air counterparts

Sonic limitations and startup problems of heat pipes

Introduction of small amounts of inert, noncombustible gas aids startup in certain types of heat pipes. When the heat pipe is closely coupled to the heat sink, the startup system must be designed to bring the heat sink on-line slowly

Heat transfer to two-phase air/water mixtures flowing in small tubes with inlet disequilibrium

The cooling of gas turbine components was the subject of considerable research. The problem is difficult because the available coolant, compressor bleed air, is itself quite hot and has relatively poor thermophysical properties for a coolant. Injecting liquid water to evaporatively cool the air prior to its contact with the hot components was proposed and studied, particularly as a method of cooling for contingency power applications. Injection of a small quantity of cold liquid water into a relatively hot coolant air stream such that evaporation of the liquid is still in process when the coolant contacts the hot component was studied. No approach was found whereby heat transfer characteristics could be confidently predicted for such a case based solely on prior studies. It was not clear whether disequilibrium between phases at the inlet to the hot component section would improve cooling relative to that obtained where equilibrium was established prior to contact with the hot surface

Jet array impingement with crossflow-correlation of streamwise resolved flow and heat transfer distributions

Correlations for heat transfer coefficients for jets of circular offices and impinging on a surface parallel to the jet orifice plate are presented. The air, following impingement, is constrained to exit in a single direction along the channel formed by the jet orifice plate and the heat transfer (impingement) surface. The downstream jets are subjected to a crossflow originating from the upstream jets. Impingement surface heat transfer coefficients resolved to one streamwise jet orifice spacing, averaged across the channel span, are correlated with the associated individual spanwise orifice row jet and crossflow velocities, and with the geometric parameters

Multiple jet impingement heat transfer characteristic: Experimental investigation of in-line and staggered arrays with crossflow

Heat transfer characteristics were obtained for configurations designed to model the impingement cooled midchord region of air cooled gas turbine airfoils. The configurations tested were inline and staggered two-dimensional arrays of circular jets with ten spanwise rows of holes. The cooling air was constrained to exit in the chordwise direction along the channel formed by the jet orifice plate and the heat transfer surface. Tests were run for chordwise jet hole spacings of five, ten, and fifteen hole diameters; spanwise spacings of four, six, and eight diameters; and channel heights of one, two, three, and six diameters. Mean jet Reynolds numbers ranged from 5000 to 50,000. The thermal boundary condition at the heat transfer test surface was isothermal. Tests were run for sets of geometrically similar configurations of different sizes. Mean and chordwise resolved Nusselt numbers were determined utilizing a specially constructed test surface which was segmented in the chordwise direction

Jet array impingement flow distributions and heat transfer characteristics. Effects of initial crossflow and nonuniform array geometry

Two-dimensional arrays of circular air jets impinging on a heat transfer surface parallel to the jet orifice plate are considered. The jet flow, after impingement, is constrained to exit in a single direction along the channel formed by the jet orifice plate and the heat transfer surface. The configurations considered are intended to model those of interest in current and contemplated gas turbine airfoil midchord cooling applications. The effects of an initial crossflow which approaches the array through an upstream extension of the channel are considered. Flow distributions as well as heat transfer coefficients and adiabatic wall temperatures resolved to one streamwise hole spacing were measured as a function of the initial crossflow rate and temperature relative to the jet flow rate and temperature. Both Nusselt number profiles and dimensionless adiabatic wall temperature (effectiveness) profiles are presented and discussed. Special test results which show a significant reduction of jet orifice discharge coefficients owing to the effect of a confined crossflow are also presented, along with a flow distribution model which incorporates those effects. A nonuniform array flow distribution model is developed and validated

Computational and Experimental Comparison of Heat Transfer Characteristics of a Triple Row Impingement Channel at Large Impingement Heights

Impingement cooling is used in a variety of applications that require high localized convection heat transfer coefficients. Turbine airfoils can be cooled with high heat transfer impingement channels, local convection coefficients are the result of both the jet impact, as well as the channel flow produced from the exiting jets and the complex interaction between the jet, the crossflow and side walls. Numerous studies have investigated the effects of jet array and channel configurations on both target and jet plate heat transfer coefficients. However, it is important to understand how the jets behave at different impingement heights in order to properly design a near-wall cooling channel for a turbine airfoil. At large impingement heights, the jets have to travel further in order to impinge on the target plate, providing the cross flow more time to deflect it downstream. This downstream deflection reduces heat transfer; however, it provides uniform cooling throughout. The present study compares effects of jet-to-target wall distance and jet Reynolds number on local variation of the heat transfer coefficient. Results are presented for average jet based Reynolds numbers between 7,500 and 15,000. All experiments were carried out on an impingement channel with 3 jet holes per row, and a total of 15 streamwise rows, with X/D of 5, Y/D of 2, and Z/D of 6, 8 and 10, with a total channel width of Yc/D of 8. Results showed that the channel with a height of 6 diameters, and high Reynolds number yields highest heat transfer coefficients. The heat transfer profiles show very strong heat transfer in the first 4 rows of jets, rapidly decaying in the downstream direction due to crossflow degradation. CFD also showed the rapid degradation of heat transfer between the third and fourth row; although the magnitudes of heat transfer coefficients were upwards of 40% off from the experimental value. © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved

The mother's mark: representations of maternal influence in Middle English popular romance

This dissertation investigates fourteenth- and fifteenth-century romances in English as they struggle with the complicated question of maternal influence, collectively constructed by intersecting, yet often contradictory discourses and interests. I argue that for Chaucer and the late medieval poets who wrote Octavian, Sir Gowther, and Melusine, the genre of the family romance proved particularly conducive to exploring the status of maternal influence and contribution in the context of these political, medical and religious contexts in their poems. In this project, I argue that not only is biological maternity and its significance interrogated in these romances, but that romance, especially the so-called "family romances" that gained in popularity in the later Middle Ages, with their narrativization of the vicissitudes of genealogy, offered poets an appropriate vehicle for meditating on the problems mothers posed to patriarchal genealogies--and, in some cases, the solutions they offered. Religious and medical texts often located maternal influence as a source of deviance, even monstrosity. Yet Octavian, Sir Gowther, Chaucer's Man of Law's and Clerk's Tales, and the Middle English Melusine undermine and critique paternal claims of maternal monstrosity or pollution as both untrue and ultimately dangerous to the genealogical project of reproducing the patrilineal dynasty. Modern scholarly discussions of medieval maternity tend to avoid the maternal body itself, identifying motherhood as a series of practices or identifying maternal images and metaphors as they were used by non-reproductive figures to describe their identities in other contexts. This project seeks to shift the register of an emerging conversation about medieval maternity to a more complicated level, one which acknowledges and references the complex and ambivalent social contexts in which maternal bodies and their influence were read and interpreted in the late Middle Ages. From the Octavian-poet, who acknowledges and refutes claims that the maternal body is a source of pollution, to the Melusine-poet's examination of the repercussions of recognizing and acknowledging maternal influence, late medieval poets approached the maternal body with profound ambivalence and an awareness of the social and religious stakes involved in representing that body and its significance to the community.Ph.D.Includes bibliographical references (p. 243-259)