Heat Flux Generation Research Articles

Flame/wall interaction and maximum wall heat fluxes in diffusion burners

Determining maximum wall heat fluxes in diffusion burners (rocket engines, diesel engines, or gas turbines) is a critical question for many combustion chambers. These fluxes are often due to flame/wall interaction (FWI) phenomena. This interaction may be due to either premixed or diffusion flamelets impinging on walls. Possible FWI configurations in diffusion burners are analyzed, and a proper laminar configuration for FWI studies (a diffusion flame impinging on a wall in a stagnation point flow) is identified. This case is studied for a variety of strain rates in terms of minimum distances between flame and wall and maximum wall heat fluxes. Results show that diffusion flames interacting with walls can generate heat fluxes which are larger than a premixed stoichiometric flame and that these mechanisms must be accounted for to predict wall heat fluxes in real burners.

A comparison between a small in-line and a staggered tube bank condensing steam filmwise at low-pressures

Abstract Data have been produced for filmwise condensation of steam, and steam–air mixtures, flowing downwards across two tube bank, a 15 row, in-line bank containing 75 tubes and a 15 row staggered bank containing 82 tubes. Both banks were tested at conditions typical of those found in the UK electricity generating industry. Steam was supplied at pressures of 50, 75 and 100 mbar, at velocities of 10, 20 and 33 m/s and with air concentrations of 0 and 10,000 ppm. Steam to cooling water temperature differences of 5, 10 and 15 K were used to generate heat fluxes of up to 90 kW / m 2 . The data and a mathematical model were used to investigate the effect of geometry difference on the heat transfer and pressure difference characteristics. For this particular configuration, the staggered and the in-line tube banks gave the same performance. This is not consistent with similar studies done by other researchers and indicates that tube spacing is important.

An experimental study of low-pressure filmwise condensation on a small in-line tube bank

A purpose-built test facility has been constructed and used to produce data for filmwise condensation from steam, and steam-air mixtures, flowing downwards across a 15 row by 5 column bundle of tubes. Data were obtained at conditions typical of those found in the UK electricity generating industry. Steam was supplied at pressures of 50, 75 and 100 mbar, at velocities of 10,20 and 33 m/s and with air concentrations of 0 and 10000 ppm. Steam to cooling water temperature differences of 5, 10 and 15 K were used to generate heat fluxes of up to 90 kW/m2. The data were used to investigate the application to tube bundle design. It is shown that the selection of an appropriate steam velocity allows good agreement between some shell-side heat transfer correlations and the experimental data, and that existing correlations for inundation and air-concentration effects are sufficient.

Effect of heated length on the critical heat flux of subcooled flow boiling. Part 2: Effective heated length under axially nonuniform heating condition

The effect of heated length on the Critical Heat Flux (CHF) of subcooled flow boiling with water was experimentally investigated by using a direct current heated tube made of stainless steel, a part of whose wall thickness was axially made thin for realizing nonuniform heat flux conditions. The higher enhancement of the CHF was derived for the shorter tube length. The effective heated length was determined for the tube under axially nonuniform heating conditions. When a lower heat flux part below the Net Vapor Generation (NVG) heat flux exists at the middle of the tube length, the effective heated length becomes the tube length downstream of the lower heat flux parts. However, when the lower heat flux part is above the NVG, the effective heated length is the full tube length. © 2000 Scripta Technica, Heat Trans Asian Res, 29(2): 144–154, 2000

Effect of heated length on the critical heat flux of subcooled flow boiling. Part 2: Effective heated length under axially nonuniform heating condition

The effect of heated length on the Critical Heat Flux (CHF) of subcooled flow boiling with water was experimentally investigated by using a direct current heated tube made of stainless steel, a part of whose wall thickness was axially made thin for realizing nonuniform heat flux conditions. The higher enhancement of the CHF was derived for the shorter tube length. The effective heated length was determined for the tube under axially nonuniform heating conditions. When a lower heat flux part below the Net Vapor Generation (NVG) heat flux exists at the middle of the tube length, the effective heated length becomes the tube length downstream of the lower heat flux parts. However, when the lower heat flux part is above the NVG, the effective heated length is the full tube length. © 2000 Scripta Technica, Heat Trans Asian Res, 29(2): 144–154, 2000

Enhancement of subcooled flow boiling critical heat flux for water in tubes with internal twisted tapes under one-sided-heating conditions

The heat flux distributions, q w, at the inner cooled wall for the beam irradiated test data using swirl tubes were estimated by solving an unsteady heat conduction equation. The results were compared with the net vapor generation heat flux, q NVG. It was confirmed that the lowest heat flux at the cooled wall was lower than q NVG for all data and this fact might enhance the critical heat flux (CHF) under non-uniform heating conditions. Enhancement of the CHF for the beam irradiated test data was closely connected with the region at q W> q NVG and the CHF multiplier could be related to the non-uniformity factor.

Multilayer analysis of anisotropic heat flux in vertical cavity-surface emitting lasers with quarter-wave semiconducting mirrors

Monolithic arrays of vertical cavity-surface emitting lasers (VCSELs) have the potential to be used in microwave systems based on opto-electronic technologies. However, the temperature has a strong influence over several characteristics of VCSELs. Self-heating may restrain the gain inside the device cavity and it is responsible for an increase in the laser threshold current as well as a decrease of its output power. In this paper, the thermal behavior of VCSELs was evaluated by means of a procedure that takes into account the multilayer aspects of the heat-flux propagation. The method has the advantage of dealing with the heat propagation inside each layer of the periodic structures of the device. Thus, the different characteristics and influence of the materials employed in the chip manufacturing can he considered and the device temperature profile can be predicted. From the method assumptions and simulations, the thermal resistances of typical devices were calculated. The results were shown to be in good agreement with experimental values reported in the literature.

Critical heat flux in subcooled flow boiling with water in a tube with axially nonuniform heating

Critical heat flux (CHF) in subcooled flow boiling under axially nonuniform heating conditions was experimentally investigated using a tube heated with a dc power source. The thickness of the tube wall in the axial direction was varied to attain axially nonuniform heating. The different thicknesses, therefore, separated the tube into regions of high heat flux and regions of low heat flux. The lengths of these regions of the tube were also varied to study the effect on the CHF. The objective of this system is to initiate boiling in the high-heat-flux region, thus increasing heat transfer, and to interrupt the bubble boundary layer in the low-heat-flux region. Because it is the initiation of boiling that increases heat transfer, the performance of such a system is linked to its effectiveness in repeatedly interrupting and re-establishing the bubble boundary layer. Our experiments, involving tubes that had sections of different thicknesses and different lengths, showed that when the heat flux in the low-heat-flux region was below the net vapor generation (NVG) heat flux, this system enhanced the CHF, but not when it was above the NVG. Also, for relatively short low-heat-flux regions, the CHF was not enhanced, presumably because there was insufficient time to interrupt the bubble boundary layer. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(2): 169–178, 1998

Effect of Heated Length on the Critical Heat Flux of Subcooled Flow Boiling. 2nd Report. Effective Heated Length under Axially Nonuniform Heating Condition.

Effect of heated length on the Critical Heat Flux (CHF) of subcooled flow boiling with water was experimentally investigated by using direct current heated tube made of stainless steel a part of whose wall thickness was axially cut for realizing nonuniform heat flux condition. The higher enhancement of the CHF was derived for shorter tube length. The effective heated length was determined for the tube under axially nonuniform heat flux condition. When the lower heat flux part below the Net Vapor Generation (NVG) heat flux exists at the middle of tube length, then the effective heated length becomes the tube length downstream the lower heat flux parts. However, when the lower heat flux part is above the NVG, then the effective heated length is full tube length.

An experimental study of heat transfer in a simulated turbine blade cooling passage

This paper describes an experimental investigation of heat transfer inside a simulated cooling channel for a gas turbine rotor blade. The channel is circular in cross-section and rotates about an axis which is orthogonal to its centre line. The study is aimed at the development of an experimental procedure and method of data processing which permits the determination of full axial and circumferential heat transfer data over the tube's inner surface. This is referred to as full field heat transfer data. In this respect a study of the combined effect of Coriolis and centripetal buoyancy forces on the forced convection mechanism inside the tube is the strategic aim. The experimental technique involved the determination of the inside surface temperature and heat flux distribution using a solution of the channel wall heat conduction equation. A series of wall temperature measurements on the leading and trailing edges of the channel, together with a prescribed electrically generated heat flux on the external surface, were used as boundary conditions with which to solve the channel wall heat conduction equation. The resulting internal heat flux distribution over the full inner surface was subsequently used to determine the local variation of heat transfer coefficient. The method was validated using data available in the technical literature and subsequently used to study the individual effects of Coriolis induced secondary flow and centripetal buoyancy using new data generated for the investigation.

Reynolds number dependence of the drag coefficient for laminar flow through electrically-heated photoetched screens

Experiments are performed to measure the drag coefficient of electrically-heated screens. Square-pattern 80 mesh and 100 mesh screens of 50.8 μm-wide wires photoetched from 50.8 μm thick Inconel sheets are examined. Ambient air is passed through these screens at upstream velocities yielding wire-width Reynolds numbers from 2 to 35, and electrical current is passed through the screens to generate heat fluxes from o to 0.17 MW/m2, based on the total screen area. The dependence of the drag coefficient on Reynolds number and heat flux is determined for these two screens by measuring pressure drops across the screens for a variety of conditions in these ranges. In all cases, heating is found to increase the drag coefficient above the unheated value. A correlation relating the heated drag coefficient to the unheated drag coefficient is developed based on the idea that the main effect of heating at these levels is to modify the Reynolds number through modifying the viscosity. This correlation is seen to reproduce the experimental results closely.

Current distribution in rail launchers via an equivalent network simulation approach

An equivalent network approach is presented for the evaluation of the current distribution in the rails of electromagnetic launchers. The massive conductors are simulated by means of a passive network mesh, constituted by resistive and inductive nonlinear and time-varying parameters. The moving plasma-armature is simulated by means of nonlinear active dipoles. The increase of the electrical resistivity due to the ohmically generated heat flux is taken into account in the calculation. Finally, the electric and thermal stresses in the rails are reported. >

Thermo-optical Interactions in Optical Fibres Caused by Axial Heat-flux Propagation

Abstract Mode distribution changes were observed when fibre cladding was locally heated by a focused c.w. laser. The time-evolution of mode-coupling was experimentally obtained with the help of a linear photodiode array. A simplified theoretical model of fibre axial temperature flow gave a good agreement with experiment.

Thermal wave propagation within a medium with the inert heat source

A heat conduction equation of a new type is derived which takes into account the finite velocity of heat flux propagation and the relaxation of heat source capacity. The equation is solved for a semi-infinite body and a step change in temperature at the surface. The analysis shows that as the time increases the obtained solution moves from the solution of the classical hyperbolic equation without energy generation towards the solution of the classical hyperbolic equation with energy generation.

Combustion of a carbon surface in a stagnation point flow field part II: Ignition and quench phenomena

This paper presents results of a theoretical investigation of the combustion of a carbon surface in an oxidizing cross flow in the stagnation point region. A preceeding study by the authors, which has mainly been concerned with the influence of the free stream parameters on the rate of combustion, is extended to the investigation of ignition and quench phenomena leading to discontinuous behavior of the steady state solutions. With respect to experimental studies of the stagnation point combustion of carbon, which are based on different mechanisms of controlling the combustion process, two of these mechanisms are distinguished in this paper. On the one hand an externally generated heat flux to the surface may control the process, and on the other hand the surface temperature may be the controlling or independent variable. The ignition and quench behavior strongly depends upon these controlling mechanisms. In the first case up to five steady state solutions may be obtained corresponding to different heterogeneously or homogeneously ignited or quenched states. If the surface temperature is the controlling parameter no heterogeneous ignition or quenching occurs. Then only up to three steady state solutions are possible, which correspond to an ignited or quenched homogeneous reaction. The sensitivity of the ignition and quench behavior to variations in the free stream parameters and in the heterogeneous reaction rate data is investigated with respect to both controlling mechanisms. Emphasis is put on the influence of the Boudouard reaction rate data, which are rather uncertain.

Geothermal measurements in five small lakes of northwest Ontario

The heat flow through the floors of five small lakes of known thermal history on the Canadian Shield was measured with a modified Bullard probe. A small correction for seasonal bottom water temperature variations was applied to temperature gradient measurements, and the heat flows are corrected for glaciation, lateral temperature gradients, sedimentation rates, and lateral thermal conductivity changes. Four lakes have an average heat flow of 49 ± 4 mW/m2 (1.2 ± 0.1 μcal/cm2 s). A high heat flow in the fifth lake is thought due to unusual refraction effects. The heat generation–heat flux combination yields a point that falls near accepted lines for the Canadian Shield.

Heat Transfer in Turbulent Flow with Internal Heat Generation in Concentric Annulus, (II)

The previous paper(1) dealt with the problem of fully developed heat transfer in a turbulent flow with uniform internal heat generation in a concentric annulus under conditions of uniform but different heat fluxes at the two wall surfaces. This sequel presents analyses of the heat transfer problem in the thermal entrance region. First, a graphical representation is given of the variations of Nusselt number at each wall surface with distance along the passage. Second, these fundamental solutions for uniform internal heat generation and uniform wall heat fluxes are applied to the case of arbitrary variations of internal heat generation and wall heat flux along the axial direction.

Heat Transfer in Turbulent Flow with Internal Heat Generation in Concentric Annulus, (II)

The previous paper(1) dealt with the problem of fully developed heat transfer in a turbulent flow with uniform internal heat generation in a concentric annulus under conditions of uniform but different heat fluxes at the two wall surfaces. This sequel presents analyses of the heat transfer problem in the thermal entrance region. First, a graphical representation is given of the variations of Nusselt number at each wall surface with distance along the passage. Second, these fundamental solutions for uniform internal heat generation and uniform wall heat fluxes are applied to the case of arbitrary variations of internal heat generation and wall heat flux along the axial direction.