Instantaneous Heat Transfer Research Articles

A transducer for the measurement of instantaneous local heat flux to surfaces immersed in high-temperature fluidized beds

An abrasion resistant, fast responding (settling time approximately 5 ms) heat flux transducer with associated analog signal conditioning equipment has been developed for the measurement of instantaneous local heat transfer rates to surfaces immersed in fluidized beds at combustion-level temperatures. Analog signal conditioning is used to provide a d.c. voltage which is linearly related to the instantaneous local heat flux. This voltage is suitable for direct oscillographic recording or digital data acquisition. Instantaneous local bed-to-wall heat flux data obtained in a small (102 × 102 mm) electrically heated fluidized bed at 282°C are presented.

TEMPORAL AND SPATIAL CHARACTERISTICS OF HEAT TRANSFER AT THE REATTACHMENT REGION OF A BACKWARD-FACING STEP

Time-serial data were accumulated for the distribution of instantaneous heat transfer coefficients around the flow reattachment region behind a two-dimensional backward-facing step, in order to clarify the heat transfer mechanism at that region. Ten heat flux meters, based on a constant-temperature method, were arrayed on the bottom surface of the step. The ensemble-averaged heat transfer coefficients were calculated under the condition that the maximum heat transfer coefficient appeared at a specified heat flux meter. On the average the peak value of the instantaneous heat transfer distribution shows a maximum at about one step height upstream position from the time-averaged reattachment point. In general, the position of the maximum instantaneous heat transfer coefficient moves downward and appears discontinuous^ near the time-averaged reattachment point.

Pulse Tube Refrigeration–A New Type of Cryocooler

A variation of the Stirling cryocooler, known as a pulse tube refrigerator, requires only one moving part - a pressure wave generator at room temperature. The Stirling cryocooler requires in addition a moving displacer at low temperatures to cause a phase shift between pressure and mass flow rate that leads to a refrigeration effect. This paper generalizes the Stirling cycle and shows how instantaneous heat transfer or flow through an orifice can cause a similar phase shift without a moving displacer. Temperatures as low as 60 K have been achieved in a one-stage device where an orifice is used for the phase shifting. The heat transfer mechanism for phase shifting does not yield as low a temperature, but it is semireversible and was used by Wheatley to convert heat energy to acoustic energy, which in turn drove a thermoacoustic refrigerator with no moving parts. This paper compares various pulse tube refrigerators and the Stirling refrigerator using a newly developed enthalpy flow model. Some generalizations of these concepts to systems other than gas systems are made.

Direct contact heat transfer with phase change: Bubble growth and collapse

AbstractA theoretical expression has been presented for the prediction of the instantaneous heat transfer coefficient of a condensing two‐phase bubble. The modified form of this model predicts very well the available experimental data. Another expression, presented for the total time taken by an evaporating two‐phase drop, also gives very good agreement with the available experimental data.

A study of the time and spatial micro-structure of heat transfer performance near the reattaching point of separated flows.

An experimental study is made for the separated an reattached flow behind a backward facing step to clarify time-mean and instantaneous heat transfer performances near the reattaching point. The reattaching point is found to fluctuate almost in accordance with the normal distribution. By using several time-mean heat transfer experimental results of different fluctuation amplitude, the instantaneous heat transfer performance near the reattaching point is obtained. Based on this instantaneous performance, the time-mean heat transfer characteristic is calculated and found to be in good agreement with the experimental results.

Unsteady Aerodynamic and Heat Transfer Processes in a Transonic Turbine Stage

The effect of the interaction of the wake from a nozzle guide vane with the rotor may be simulated in part by means of a stationary rotor and a moving wake system. This technique is applied to a transonic rotor blade cascade, and the unsteady measurements of surface pressure and heat transfer rate are compared with baseline data obtained without the wake interaction. The wake-rotor interaction results in a change in inlet incidence angle and this effect is also examined in the steady-state case. It is found that the shock waves from the moving wake system have a major effect on the instantaneous heat transfer rates.

Convective Heat Transfer in Reciprocating Engines

A formula is presented for computing the instantaneous convective heat transfer coefficient between the charge and the confining walls in a reciprocating engine. The formula is developed from basic considerations of turbulent diffusion and relates the convective heat transfer coefficient directly to the turbulence intensity in the charge. This feature distinguishes it from the existing methods of computing the convective heat transfer coefficient which require a precise specification of the continually changing velocity and length scales of the fluid flow. The proposed formula is therefore expected to be more accurate in heat transfer modelling for new and existing engines, particularly as turbulence measurements in engines progressively improve.

Liquid Crystal Visualization of Surface Heat Transfer on a Concavely Curved Turbulent Boundary Layer

An experimental heat transfer study on a concavely curved turbulent boundary layer has been performed. A new, instantaneous heat transfer measurement technique utilizing liquid crystals was used to provide a vivid picture of the local distribution of surface heat transfer coefficient. Large scale wall traces, composed of streak patterns on the surface, were observed to appear and disappear at random, but there was no evidence of a spanwise stationary heat transfer distribution, nor any evidence of large scale structures resembling Taylor-Gortler vortices. The use of a two-dimensional computation scheme to predict heat transfer rates in concave curvature regions seems justifiable.

Turbulent Heat Transfer Associated with Coherent Structures in Fully Developed Pipe Flow

Turbulent heat flux and Reynolds stress fluctuations in fully developed pipe flow have been measured by employing the combined cold wire and V-shaped hot wire anemometry technique, and then conditionally sampled and averaged. Instantaneous turbulent heat transfer is strongly associated with and dominated by the coherent turbulent structures, and thus the inherent intermittency of turbulent heat transport process is the consequence of the intermittent coherent motion near the wall. Conditionally averaged patterns of velocity, temperature and relevant turbulent heat flux fluctuations are obtained by analyzing the amplitude, mean period and duration of coherent motions. These recognized patterns reveal how the turbulent heat transfer is affected by four distinct types of coherent motions originating in the wall region and extending through a greater of the pipe section.

An experiment on transient heat and mass transfer in humid air flow

This paper reports the results of a preliminary investigation of transient condensation and evaporation for humid air flow. Using the principle of Newtonian heating, a novel experimental procedure has been developed for the determination of the instantaneous heat transfer coefficients and mass transfer rates and their dependence on time. Tests were made on a pre-cooled cylinder of negligible thermal resistance in a cross flow of humid air over a Reynolds number range 9300–22200. Provisional comparison with previous data for condensation in the steady-state case has been made and indicates that the proposed method may prove a valuable alternative to the steady-state technique of measurement in this area of study.

An experimental and analytical study of heat transfer in an engine exhaust port

Because of the increasing need for precise information on engine processes, this study developed and experimentally verified models for the instantaneous heat transfer in an engine exhaust port. Experimental measurements were made of the instantaneous cylinder pressure and instantaneous exhaust gas temperature for a range of engine operating conditions. The pressure measurements were used to obtain the instantaneous cylinder gas state and the temperature measurements were used to validate the heat transfer models. The exhaust port heat transfer was dominated by jet induced large-scale fluid motion as opposed to wallshear generated fluid motion. An analysis based on the jet velocity through the valve opening correctly estimated the heat transfer due to this large-scale motion. Models for the instantaneous exhaust port heat transfer provided excellent agreement with measurements as a function of engine operating conditions.

A NUMERICAL METHOD FOR DETERMINING DIRECT-CONTACT HEAT TRANSFER RATES TO A SUPERHEATED EVAPORATING FLOATING DROPLET

A mathematical model of a single evaporating, floating, immiscible, superheated droplet is developed. The key to the model is the validity of the kinetic theory-derived Knudsen equation when applied to superheated liquids. By employing an iterative finite-difference scheme, the model may be analyzed numerically to determine instantaneous and average heat transfer rates to the droplet at various stages of evaporation. Sample results represent n-pentane droplets evaporating from a water surface.

Latent Heat-of-Fusion Energy Storage: Experiments on Heat Transfer from Cylinders During Melting

Melting from an array of three staggered, electrically heated cylinders imbedded in a paraffin (n-octadecane) has been studied. The shape of the melting front has been determined photographically, and the local heat transfer coefficients were measured using a shadowgraph technique. The experiments provide conclusive evidence of the important role played by natural convection on the timewise variation of the melt shape, the surface temperature and the instantaneous local as well as circumferentially averaged heat transfer coefficients around the imbedded heat sources. After a common solid-liquid interface is formed around the cylinders, natural convection circulation around each cylinder interacts strongly with the other two cylinders. The arrangement of heat sources affects significantly the melt shape but the circumferentially averaged instantaneous heat transfer coefficients differ only by about 10 percent for the two arrangements studied. The experimental findings indicate that natural convection effects are important and should be considered in analysis and design of systems involving phase change.

Local heat transfer coefficients in scraped-film heat exchanger.

The instantaneous heat transfer coefficients in a scraped-film heat exchanger have been determined using the electrochemical technique for a two-bladed, 78.7mm I.D. exchanger. The time-averaged local heat transfer coefficients were found to agree fairly well with previous work of Azoory and Bott, though some variation in liquid flow rate was detected. The timeaveraged local Nusselt number was found to be represented by Nu=Q.15(Rer•Pr)1/2Reawwith a=(l-3.74×10-2N)/9. The variation of heat transfer coefficient with time was observed to be very different from that predicted by the accepted model for the phenomena based upon unsteady state conduction.

Interferometric measurement of instantaneous local heat transfer from a horizontally vibrating isothermal cylinder

Local instantaneous heat-transfer coefficients were measured for free convection from an isothermal cylinder vibrating sinusoidally in a horizontal plane. The results show that the heat-transfer coefficient is strongly dependent upon the ratio of amplitude of vibration to cylinder diameter, A D . For values of A D less than approximately 0·25, a critical vibration intensity was observed below which vibration caused only small increases in the heat-transfer rate; however, at A D greater than 0·25, the critical vibrational intensity did not appear to exist because the heat transfer from the cylinder increased as soon as the vibrational speed increased from zero. A correlation of the data is presented which predicts the increase in average heat transfer from the cylinder to air above the free convective rate. The correlation also applies to data for much larger vibrational intensities.

Transient film and transition boiling from a sphere

The nature of transition boiling from 3 4 in. dia. copper sphere moving through subcooled distilled water was experimentally investigated. High-speed photographs show that transition occurs as a rapidly pulsating vapor film on the forward portion of the sphere. The behavior of the vapor wake is highly dependent upon the manner in which vapor is being formed on the forward portion of the sphere. Instantaneous heat transfer rates calculated from experimental temperature-time measurements indicate that film and transition boiling are as effective as nucleate boiling over the velocity range, 9.6–20.0 fps, in highly subcooled water. That is, there is no significant change in the heat transfer rate at transition as observed for free convection quenching of spheres.

Instantaneous Measurement of Heat Transfer From an Oscillating Wire in Free Convection

The instantaneous heat transfer properties are measured as a function of time for an oscillating wire (20 to 40 Hz) in still air. The wire is oscillated by thermal contractions and expansions which match the natural frequency based on wire mass and tension. The temperature variation results from the internal resistance heating of an alternating current. The wire temperature and velocity are measured as a function of time by photocells. This eliminates any instrumentation interference with the heat transfer. The results are plotted as a function of instantaneous and average Reynolds’ number. The oscillatory heat transfer data are divided into two regimes of free and forced convection by the critical Reynolds number. Oscillatory heat transfer rates are smaller for forced convection and greater for free convection than those for steady state conditions recommended by McAdams [2] for the respective regimes. No significant difference is found in the heat transfer for oscillations in the vertical and horizontal planes. Due to the time variation of the variables an appreciable amount of emphasis is placed on the experimental apparatus and the recording of data. The recorded data is basically corrected by assuming first order linear systems.

Second Paper: Instantaneous Heat Transfer Rates to the Cylinder Head Surface of a Small Compression-Ignition Engine

Measurements of instantaneous heat transfer rates from the working fluid to the cylinder head of a small open-chamber, four-stroke, compression-ignition engine have been made at five points on the surface, using a new form of thin-film thermocouple. These observations demonstrate that flux magnitude and the form of flux variation during the cycle depend greatly on local conditions. Some of the observed phenomena are explained qualitatively. Finally, some results are presented of an analysis of fluxes averaged over all five locations, in terms of the bulk mean properties of the working fluid. It is shown that some compensation for the non-steady nature of the situation may be given by adding to the usual type of quasi-steady relation a term involving the time derivative of the bulk mean temperature.

酸素アセチレン焔の熱効率ならびに熱伝達率について(第3報)

The end of steel bar specimens with various diameter have been heated by mean of oxy-acetylene tips (capacity : 500;) with single and multi nozzles that is nearly equal to flat theamic source as is shown in Fig. 5, in order to measure the thermal efficiency of oxy-acetylene flame and the heat transfer coefficient between heated surface of bar specimen and the flame.The test results can be described as follows ;1) The heat quantity corresponded to 35% in total transfered heat quantity was given from the front or top of white cone to the specimen. This decision was based on the test results as is shown in Fig 1, 2, 4 and 7, and the result of consideration for them.2) A few matters should be studied are remained, however, the authors introduced the formulas (1) and (2) on the instantaneous and mean heat transfer coefficient between bar specimen and the front on top of white cone in the heated period. In this case, the temperature in front or top of white cone was assumed 3, 200°C.