Instantaneous Heat Flux 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.

A study of the local heat transfer coefficient on the combustion chamber walls of a 4-stroke gasoline engine.

It is said that coefficients of heat transfer on combustion chamber walls of internal combustion engines differ according to individual locations, but no actual measurements have been reported. This study determined the coefficient of local heat transfer at each location on the wall surfaces of a combustion chamber obtained by the instantaneous temperature Tw and the instantaneous heat flux q at each location measured by highly accurate thin film thermocouples developed by the authors, and the instantaneous gas temperature Tg in the combustion chamber determined by the gas pressure P in the combustion chamber, the only source of information on gas in the chamber. As a result, it was found that the engine speed, gas flow, ignition timing, distance from the plug, shape of combustion chamber, temperature, etc. would affect significantly on the local heat transfer coefficient. It was also found that Eichelberg's equation or Woschni's equation in which the gas flow is taken into account would yield substantial differences from measured values in the high engine speed range.

Instantaneous heat flux on the piston crown of a small size direct injection diesel engine (Effects of gas flow and fuel spray jet)

The effects of gas flow (swirl and squish) and fuel spray jet on local instantaneous heat flux on the piston crown of a small size direct injection diesel engine are experimentally investigated. Under the motoring operation, local heat flux on the piston with a flat crown increases with increasing local swirl velocity; and the local Nusselt number is found to be proportional to R<0.636gt;e, where Re=2πrvθ, r/ν with vθ, r the local swirl velocity at radius r andνthe kinematic viscosity. While for the piston with cavity, local heat flux is strongly influenced by squish flow. Under the firing operation, it is observed that the heat flux at the position where the fuel spray jet collides is mostly determined by the fuel spray jet.

Heat Transfer to Wall of Ceramic Combustion Chamber of Internal Combustion Engine

Thin film thermocouples have been successfully produced, using ceramics as the body material, through the application of the technique used to make thin film thermocouples previously developed by the authors. The measurements of instantaneous temperature and instantaneous heat flux on the surface of the ceramic plate fixed on top of the piston were made, using the thin film thermocouples thus developed. As a result, conditions of the heat transfer to the ceramic wall of high temperature have been established.

Heat Loss into Combustion Chamber Wall of 4-stroke Gasoline Engine : 2nd Report, Heat Loss into Cylinder Head, Intake and Exhaust Valves

Following the surface temperature variations and instantaneous heat flux in each cycle in the piston and the cylinder wall mentioned in the first report, the authors tried to verify them in the cylinder head and the suction/exhaust valve of a 4-stroke gasoline engine combustion chamber. The authors could make clear the heat flux distribution and heat loss conditions through the cylinder head and the valve surface for each stroke. In addition to the results as described in the first report, the heat loss on the entire combustion chamber wall against the quantity of supplied heat, namely, the ratio of heat transfer was known.

Study on Thin Film Thermocouple Measuring Instantaneous Temperature on Surface of Combustion Chamber Wall In Internal Combustion Engine : 2nd Report, Study on Thin Film Thermocouples Embedded in Combustion Chamber Wall

Following a study in which the area surrounding thin film thermocouples was assumed to be heat insulated as reported previously, the distribution of temperatures in thin film thermocouples embedded in combustion chamber walls of internal combustion engines which were in steady-state and unsteady-state temperature fields was studied, where the equations of different element methods, introduced in previous report, were expanded. Effects of the adiabatic layer and thermocouple body materials on the measured combustion chamber wall surface temperatures and heat fluxes were studied. As a result, it was indicated that thin film thermocouples capable of measuring instantaneous temperatures and heat fluxes on the combustion chamber wall with high accuracy, could be produced.

Heat Loss to Combustion Chamber Wall of 4-Stroke Gasoline Engine : 1st Report, Heat Loss to Piston and Cylinder

In an attempt to find out the thermal load conditions of the combustion chamber wall, by the use of precise thin film thermocouples which the authors have developed recently, surface temperature variation and instantaneous heat flux of the piston and the cylinder of a 4-stroke gasoline engine, for each stroke, are obtained. As a result, thermal load conditions of parts of the piston and the cylinder are made clear, and along with this, the ratio of heat loss is determined. Thermal load aspects under abnormal combustion conditions i.e. knocking are also crarified.

PLANAR OR RADIAL FREEZING WITH SOLID-SOLID TRANSITIONS AND CONVECTIVE HEATING ATTHE SOLID-LIQUID INTERFACE

Numerical solutions have been carried out for planar or radial freezing on a cooled wall or cylinder maintained at a uniform temperature lower than the fusion temperature. The solutions were obtained by using an implicit/explicit method developed in the preceding paper in this issue. Results are presented for the frozen layer thick-ness, the instantaneous heat flux at the cooled surface, and the time-integrated heat transfer at the surface, all as a function of time from the beginning of the freezing period. These quantities are normalized in various ways to illuminate their short-time and long-time behaviors and to enable comparisons between planar and radial freezing. Convection at the solid-liquid interface had virtually no effect on the freezing process at short times, but at longer times the convection significantly slowed the freezing and ultimately terminated it. Solid-solid transitions tended to slow the freezing and also to increase the heat transfer at the cooled surface.

The Measurement of Vertical Transports of Sensible Heat within and above a Sorghum Canopy

The observations of sensible heat flux were made at the 3 heights (260cm, 160cm, 100cm) in the sorghum field with a mean plant height of 160cm in August, 1982. The fluctuations of air temperature were measured at the 5 heights (260cm, 160cm, 100cm, 65cm, 20cm). The results are summarized as follows.1) The fluxes of sensible heat in the daytime were upward above and within plant canopies. The sensible heat flux above plant canopies were provided about 60% of flux from the upper layer between 160cm and 100 cm and about 40% from the layer below 100cm.2) The non-dimensional standard deviations (σT/|T*|) of air temperature were maximum at the top of the plant canopies, where T*=wT/u*, w and T are fluctuations of vertical velocity and temperature, and u* the friction velocity. The skewness factors (ST) of air temperature tended to become positive above plant canopies and negative within plant canopies. The flatness factors (FT) scatters around 3 through the air layer above and within plant canopies.3) The non-dimensional standard deviations (σw/u*) of vertical velocity had smaller values within plant canopies, compared with those above plant canopies. Within plant canopies, skewness factors (Sw) were negative and flatness factors Fw were greater than 4. The height dependency was in good agreement with that of previous results (Wilson et al. (1982) and Maitani and Seo (1983)).4) The correlation coefficients (rwT) between vertical velocity and air temperature were about 0.3 at the three heights of 260cm, 160cm and 100cm. On the other hand, correlation coefficients (rw·wT) between vertical velocity and instantaneous sensible heat flux were negative within plant canopies. The negative correlation coefficients rw·wT and joint probability densities (w-wT) within plant canopies implys that downdrafts were much efficient for a upward heat flux in the daytime.5) The peak frequencies of power spectrum (nST(n)) of T and cospectrum of (nCowT(n)) at the top of the plant canopies were higher than those within plant canopies. The shapes of nST(n) and nCowT(n) become flatter with decreasing heights within plant canopies.

Wave-Induced Effect on the Reynolds Shear Stress and Heat Flux in the Marine Surface Layer

Abstract The influence of surface waves on the Reynolds shear stress and the heat flux, measured in the atmospheric surface layer ∼5 m above the ocean, is discussed using the method of Lu and Willmarth (1973). During the observational period, the phase velocity C associated with the dominant wave frequency is 33–80 times larger than the friction velocity u*. When contributions to the momentum flux −uw are sorted out into the four quadrants of the (u,w) plane, the contribution from the interaction quadrants (u>0, w>0; u<0, w<0) increases as C/u* increases. The contributions to the heat flux are not appreciably affected by C/u*. While the probability that sweep and ejection quadrants associated with instantaneous shear-stress and heat-flux fluctuations occur at the same time is large and approximately independent of C/u*, the probability of simultaneous occurrence of interaction events increases as C/u* increases. The period between ejection events also increases with C/u*.

Similarity of atmospheric Reynolds shear stress and heat flux fluctuations over a rough surface

Horizontal u and vertical w velocity fluctuations have been measured together with temperature θ fluctuations in the atmospheric surface layer, at a small height above a wheat crop canopy. Marginal probability density functions are presented for both individual fluctuations u, w, θ and for the instantaneous Reynolds stress uw, and heat fluxes wθ and uθ. Probability density functions of the velocity fluctuations deviate less significantly from the Gaussian form than the probability density of temperature. There appears to be closer similarity between statistics of the instantaneous heat fluxes than between the momentum flux and either of the heat fluxes investigated. The mean momentum flux receives equal contributions from the events referred to as ‘ejections’ and ‘sweeps’ in laboratory boundary layers. ‘Sweeps’ provide the largest contribution to the heat fluxes.

An experimental study of turbulent convective heat transfer from a flat plate

A turbulent boundary layer developing on a smooth heated uniform-temperature plate in a zero pressure gradient was set up. The origins of the layers were matched to remove the effect of an In heated starting length. Similarity proposals were tested. The mean flow field followed the usual law of the wall and defect law for both temperature and velocity. Broad-band measurements of stream wise velocity and temperature fluctuations were made, and wall similarity and Townsend's self-preserving flow similarity were found to be applicable, at least after a sufficient flow development.Some initial attempts to arrive at a comparison between heat and momentum transport are presented. The results include conditionally sampled measurements of instantaneous heat and momentum fluxes and correlations between these two quantities. The fluxes were divided into quadrants. Conditional probabilities and weighted joint probability density functions were measured to determine whether there was a similarity in behaviour of these two fluxes. The concept of ‘hole size’ developed for momentum flux was extended to heat flux and events corresponding to bursts and sweeps in the momentum flux were found to be accompanied by corresponding events in the heat flux.

Destabilization of vapor film boiling around spheres

Energy transfer and vapor film destabilization processes have been studied in an experiment where a sphere traveling with a constant velocity of 1.52 m/s was quenched in distilled water. Data were obtained for the instantaneous heat flux and the transient vapor film behavior over a range of sphere temperatures from 260 to 538 C and water subcoolings from 24 to 77 C. The nature of the transient vapor film behavior was studied with high-speed photography during quenching. All experiments were performed at atmospheric pressure. Two distinctive mechanisms for the destabilization of a stable vapor film were identified. One, a precipitous collapse and the other, progressive instability. The precipitous collapse occurs very rapidly, on the order of 0.25 ms, while the progressive instability requires from 50 to 100 ms for completion. The progressive instability is triggered by small bubble-like irregularities that move on the liquid-vapor interface. Precipitous instability appears to be dependent on heat flux rather than sphere temperature.

Magnetohydrodynamic Free Convection Due to an Instantaneous Heat Source

The effect of a uniform normal magnetic field upon the free convection from a vertical wall is considered when an instantaneous heat flux is released from the wall. Solutions for small and large times are obtained, using Laplace transform technique. In the ultimate state, a steady streaming, confined within the Hartmann layer emerges. At all stages of the motion, the shearing stress at the wall is less than its corresponding value the nonmagnetic case, provided the thermal diffusivity is less than the magnetic diffusivity.

The effect of periodic shock-fronted pressure waves on instantaneous heat flux rates

Instantaneous heat flux rates were measured at the end-wall of a tube in which shock-fronted, longitudinal pressure waves were generated. Pressure oscillations with frequencies of 479–881 Hz,amplitudes up to 175 psi peak-to-peak, and average tube pressures from 40 to 190 psia were obtained. The heat flux was calculated from the surface temperature of the end-wall as measured by a thin platinum resistance film deposited on a Pyrex surface inserted into the end-wall. The instantaneous heat flux was found to be large compared to the time-average heat transfer rate. The instantaneous rates increased with increasing pressure ratio and were proportional to the square root of the product of the wave frequency and the time-averaged pressure of the oscillations. A one-dimensional model was solved numerically for the case of zero net heat transfer to the end-wall. The calculations agreed with the experimentally observed trends and correctly predicted the shape of the periodic portion of the flux.