Decrease In Temperature Difference Research Articles

Airflow pattern and temperature distribution in a typical refrigerated truck configuration loaded with pallets

This work is part of a research activity aiming to improve and to optimise air-distribution systems in refrigerated vehicles in order to decrease the temperature differences throughout palletised cargos. This condition is essential in order to preserve the quality, safety and shelf life of perishable products. The present study reports on the numerical and experimental characterization of airflow within a semi-trailer enclosure loaded with pallets. The experiments were carried out on a reduced-scale (1:3.3) model of a refrigerated-vehicle trailer. The performance of ventilation and temperature homogeneity were characterized with and without supply air duct systems. Both configurations are extensively used in refrigerated transport. The numerical modelling of airflow was performed using the Computational Fluid Dynamics (CFD) code Fluent and a second-moment closure, the Reynolds stress model (RSM). The results obtained using the RSM model showed good agreement with the experimental data. Numerical and experimental results clearly show the importance of air ducts in decreasing temperature differences throughout the cargo.

Photoelastic Investigation of Bimaterial Interfacial Stresses Induced by Thermal Loading

Abstract: In this paper, the photoelastic technique and finite element method were employed to investigate the interfacial stresses induced by thermal loading in bimaterial structures. By observing the photoelastic fringe patterns, in comparison with those theories developed by other investigators, severe temperature and stress gradients were found across the specimens’ heights. The maximum fringe order near the interface was six times that near the top surface of the photoelastic material when the temperature was raised to 65 °C. Furthermore, for the bimaterial structures, differences were found between measured stress fields as a result of increasing and decreasing temperature differences.

CRITERIA FOR CEILING SLOT–VENTILATED AGRICULTURAL ENCLOSURES: NON–ISOTHERMAL

Experiments using two scale–models were conducted to study non–isothermal airflow similarity in a ceiling slot–ventilated agricultural enclosure. The main criteria used for determining similarity were airflow pattern similarity, occupied zone airspeed, and temperature similarity. For agricultural ventilation issues, these were projected to be paramount to a successful scale–model study. The current study, based on past agricultural ventilation studies, focused attention on the Archimedes number (Ar) and Momentum ratio (Rm) as the similitude criteria for non–isothermal airflow. Results indicated that Ar is the appropriate similitude criteria for non–isothermal airflow patterns under strong buoyancy–affected conditions where Ar > 0.015. With increasing airflow or decreasing temperature differences, resulting in a lower buoyancy–affected air–jet, Rm with a consistent overall temperature differential was determined to be the appropriate similitude criteria. This study focused on practical considerations for conducting scale mode studies such as scale–model conditions measured in the animal occupied zone and it’s relation to expected prototype behavior.

Exergy analysis of vapor compression refrigeration systems

A computational model based on the exergy analysis is presented for the investigation of the effects of the evaporating and condensing temperatures on the pressure losses, the exergy losses, the second law of efficiency, and the coefficient of performance (COP) of a vapor compression refrigeration cycle. It is found that the evaporating and condensing temperatures have strong effects on the exergy losses in the evaporator and condenser, and on the second law of efficiency and COP of the cycle but little effects on the exergy losses in the compressor and the expansion valve. The second law efficiency and the COP increases, and the total exergy loss decreases with decreasing temperature difference between the evaporator and refrigerated space and between the condenser and outside air.

Laminar film condensation heat transfer of hydrogen and nitrogen inside a vertical tube

AbstractThere have been a number of experimental investigations on condensing heat transfer to cryogenic fluids. The investigations with nitrogen and oxygen have shown reasonable agreement between experimental data and those predicted by Nusselt's theory. On the other hand, in the previous investigations with much colder fluids, such as hydrogen, deuterium, and helium, the experimental condensing heat transfer coefficients are smaller than those predicted by Nusselt's theory, and these differences become much larger when the film Reynolds number or decreasing temperature difference across the condensate film is decreased. In the present investigation, hydrogen and nitrogen were condensed inside a vertical tube (d = 15 mm, L = 30 mm) under steady‐state conditions, respectively, and condensing heat transfer coefficients were precisely measured. From the experimental results, the condensing heat transfer coefficients for saturated hydrogen and nitrogen vapors agree with those predicted by Nusselt's theory within ±20%. The results of the present study suggest that deuterium and helium might also behave as predicted by Nusselt's theory. © 2001 Scripta Technica, Heat Trans Asian Res 30(7): 542–560, 2001

Experimental investigation on the transition to steady state of self-oscillation of a thermoacoustic prime mover

A thermoacoustic prime mover can be considered as a nonlinear system in which an initial small perturbation is amplified until nonlinear mechanisms lead to a new balanced state. A series of experiments were carried out to help understand the nonlinear mechanism. The transition process is divided into three periods: a growing period in which the pressure grows exponentially with time to its maximum; a falling period in which the pressure decreases to its minimum; and a period in which the pressure is restablished slowly until the steady state. A rapid decrease in temperature difference between the cold and hot heat exchangers is observed when the pressure arrives at the maximum. The difference between maximum pressure and steady-state pressure depends on the mean pressure in the prime mover: the higher the mean pressure, the greater the difference. A model of positive feedback amplifier with nonlinear resistance is proposed to describe this phenomenon.

Evaluation of temperature increase with different amounts of magnetite in liver tissue samples.

The biologic effects of magnetically induced heating effects using iron oxide, magnetite, were examined in vitro in liver tissue samples as a first step toward potential applications in cancer therapy. For the determination of the temperature profile around an iron oxide sample, a cylinder containing 170 mg of magnetite was constructed and placed into pureed liver tissue from pig, together with thermocouples of copper and constantan wires positioned at defined distances from it. Temperature measurements were performed during the exposure to an alternating magnetic field (frequency: 400 kHz; amplitude: approximately 6.5 kA/m) generated by a circular coil (90 mm of diameter). Moreover, variable amounts of magnetite (dissolved in approximately 0.2 mL physiologic saline) were injected directly into carrageenan gels. During the exposure to a magnetic field for 4 minutes the temperature increase was determined in the area of iron oxide deposition using a thermocouple. Additionally, variable amounts of magnetite were injected directly into isolated liver tissue samples (diameter: 20 mm; height: 30 mm) and exposed to a magnetic field for 2 minutes. The extent of the induced macroscopically visible tissue alterations (light brown colorations caused by heating) was examined by means of volume estimations. The degrees of cellular necrosis were investigated by histopathologic studies. The temperature profile around a magnetite cylinder revealed a significant decrease of temperature difference between the beginning and the end of heating, depending on increasing distance from the sample center. The extent of the temperature difference correlated with increasing heating time. No significant variations of temperature were observed at a distance of approximately 12 mm from the sample center. A good correlation (r = 0.98) between the injected amounts (31 to 200 mg) and the temperature increase since the start of heating (6.8-33.7 degrees C) in the area of iron oxide deposits was detected. The volume of damaged liver tissue was approximately seven times higher than the injected volume of iron oxide dispersion. Histologically different degrees of cellular necrosis were observed. The parameters determined in this article show that iron oxides are able to induce considerable heating effects in the surroundings. After an adequate optimization of the technical procedure, it is conceivable that heating properties of magnetites can be used in future cancer treatments.

Heat Transfer With Insulated Combustion Chamber Walls and Its Influence on the Performance of Diesel Engines

Recently great expectations were put into the insulation of combustion chamber walls. A considerable reduction in fuel consumption, a marked reduction of the heat flow to the cooling water, and a significant increase of exhaust gas energy were predicted. In the meantime there exists an increasing number of publications reporting on significant increase of fuel consumption with total or partial insulation of the combustion chamber walls. In [1] a physical explanation of this effect is given: Simultaneously with the decrease of the temperature difference between gas and wall as a result of insulation, the heat transfer coefficient between gas and wall increases rapidly due to increasing wall temperature, thus overcompensating for the decrease in temperature difference between gas and wall. Hence a modified equation for calculation of the heat transfer coefficient was presented [1]. In the paper to be presented here, recent experimental results are reported that confirm the effects demonstrated in [1], including the influence of the heat transfer coefficient, which depends on the wall temperature, on the performance of naturally aspirated and turbocharged engines.

Condensing phenomena of single vapor bubble into subcooled water. 1st report Flow visualization.

Experiments have been carried out to investigate direct contact condensation of saturated vapor bubbles introduced into a quiescent subcooled water environment. The experiments were performed for a range of pressures from atmospheric to 1 MPa, for subcooling from 10 to 70°C, and for initial bubble diameters of about 10 mm. Flow visualization by high speed motion pictures was based on a frame by frame analysis. The photographs showed that the successive shapes of the bubbles during their collapse histories proceeded from a sphere to a hemisphere, to an ellipsoid, to a sphere, and finally to collapse. Two-dimensional photographs showed that the cavities of the bubbles during their collapse histories proceeded from the bottom to the top. The time to collapse increased with increasing pressure and with decreasing temperature difference. The rising velocities of the bubbles were essentially constant, with an overall range of 20∼25 cm/s.

Deviation of freezing rate from translation rate in the Bridgman-Stockbarger technique: II. Moderate translation rates

The effect of moderate ampoule motion rates on interface position, velocityand temperature gradient in Bridgman-Stockbarger crystal growth is examined. An analytical solution for the area averaged temperature field is obtained using a regular perturbation method. The predictions of the present model are in excellent agreement with numerical calculations for a semi-infinite ampoule and are in qualitative agreement with experimental data. Simple expressions are derived for the range of validity of the analytical solution. Small ampoule velocities increase the freezing rate near the bottom of the ampoule and decrease the interface velocity near the top. The shift in interface position increases with increasing travel rate, proximity of an end of the ampoule, and decreasing temperature difference between heater and cooler. The minimum deviation of freezing rate from translation rate occurs when the ampoule is approximately centered in the apparatus. In order for this deviation to be small over a substantial portion of the ampoule, it is helpful for the ampoule to be long, the radius small, the thermal conductivity small, and the heat exchange with the furnace large.

Convection in a rotating, laterally heated annulus the wave number transitions

Abstract The radial temperature differences at which the transitions from one wave number to the next occur have been measured with either increasing or decreasing positive radial temperature gradients, at five different rotation rates, with the fluid being always in thermal equilibrium and being in contact with an upper rigid lid. Hysteresis has been observed in all wave number transitions, and also in the transition to upper symmetry. There are, nevertheless, regions in the stability diagram where the wave number is unique. There is an excluded region where the wave number four cannot be obtained through quasi-steady procedure. There is a reversal of the sense of the hysteresis of the transitions. At low ΔT, a wave number transition with increasing radial temperature difference occurs at a higher ΔT, than the same transition with decreasing temperature difference. On the other hand, at large values of ΔT, a wave number transition with increasing radial temperature difference occurs at a lower ΔT, than the same transition with decreasing temperature difference. Wave number transitions with increasing ΔT, occur spontaneously out of amplitude oscillations. Wave number transitions with decreasing ΔT, occur via slow wave splitting in association with phase modulations of the waves. The uniqueness of the wave number in the unique areas of the stability diagram has been confirmed by sudden start experiments.

Evidence of Hydrophobic Effects from Critical Phenomena

AbstractData of the temperature dependence of partial molar quantities (heat capacity and temperature derivative of partial molar volume) of perfluoromethylcyclohexane and carbon tetrachloride in a mixture of critical composition are reported. It is found that for both components the signs of the critical amplitudes of the partial molar quantities are positive. Using these data a thermodynamic relation between the critical amplitudes of specific head capacity, partial molar heat capacity, temperature derivative of specific volume and temperature derivative of partial molar volume is tested. — Analogous experiments on the system isobutyric acid/water give results which are characteristically different: (1) the sign of the critical amplitude of the partial molar heat capacity of isobutyric acid is positive whereas that of water is negative, (2) the opposite is true for the sign of the critical amplitude of the temperature derivative of the partial molar volume of the two components. The characteristics of the system isobutyric acid/water are interpreted by assuming that in a critical mixture, approaching the critical point from the one fluid phase region, the extent of hydrogen bonding between water molecules in the vicinity of the hydrophobic part of isobutyric acid molecules increases with decreasing temperature difference from the critical temperature.

Direct contact condensation of steam bubbles in water at high pressure

Experiments have been carried out to investigate direct contact condensation of saturated steam bubbles introduced into a quiescent subcooled water environment. The experiments were performed for a range of pressures from 10.3 to 62.1 bar (150–900 lb in 2 ), for subcooling from 15 to 100°C, and for initial bubble diameters of about 3mm. The data reduction of high speed motion pictures was based on a frame by frame analysis wherein the coordinates of the bubble perimeter were recorded in digital form and subsequently processed to yield quantitative information about the bubble collapse history and heat-transfer coefficient. The photographs showed that the successive shapes of the bubbles during their collapse histories proceeded from a sphere to a hemisphere to an ellipsoid to a sphere to collapse; short-lived bubbles collapsed as ellipsoids. The time to collapse and the height to collapse increased with increasing pressure and with decreasing temperature difference. The rise velocities of the bubbles were essentially constant, with an overall range of 15–22cm/s. The average heat-transfer coefficients were on the order of 10 4W/m 2·°C (1750 Btu/h·ft 2·°F), with only modest variations with pressure level and temperature difference. The instantaneous heat-transfer coefficients did not differ appreciably from the average coefficients.

Radiation Effects on Heat Transfer in the Reactor Core and Heat Exchangers of an HTGR

A general and fundamental study of the effect of radiation between duct walls on the heat transfer performance of duct flows of nonradiating gas such as helium is made by an approximate analysis and numerical calculations. Laminar and turbulent flows in a duct simulating a coolant passage in an HTGR and flows in counterflow and parallel-flow heat exchangers in an HTGR heat transfer system are investigated. An approximate analytical solution, based on the assumption that radiation from a point of duct wall produces an effect only on the narrow region opposite the point, agrees well with numerical results. The increase of radiative heat transfer causes a decrease of temperature difference between the duct walls and improves the heat transfer performance. For heat exchangers the heat transfer effectiveness is shown to depend on three nondimensional parameters and can be improved by the increase of these parameters.

Non‐Local Effects in the Diffusion Coefficient of a Binary Fluid Near the Critical Mixing Point

AbstractMeasurements are reported for the intensity and spectral width of light scattered by concentration fluctuations near the critical mixing point of the binary liquid mixture 3‐methylpentane‐nitroethane. The data were taken at the critical concentration as a function of temperature and scattering angle.The intensity data allow us to determine the susceptibility (∂c/∂ μ)T,P and the equilibrium correlation length ξ as a function of temperature. The decay rate of the concentration fluctuations, as determined from the linewidth of the central component in the spectrum of scattered light, was measured as a function of ξ and wave number k in the range 0.1 ≤ kξ, ≤ 20. In the hydrodynamic regime (kξ << 1) the decay rate Γ decreases with decreasing temperature difference Δ T =T‐Tc, but approaches a constant value in the critical regime (kξ >> 1). We have verified the prediction from dynamical scaling that Γ should vary as k3 in the limit kξ>> 1. The observed variation of Γ with k ξ is compared with the theory of Kawasaki. Differences between theory and experiment are observed if one imposes the condition that the behavior of the decay rate is governed by the same correlation length ξ that is determined by the static properties. A major part of the disagreement could be accounted for if one assumes the presence of a background term similar to the one previously introduced in the thermal diffusivity near the gas‐liquid critical point. It seems likely, however, that the decay rate is also affected by the anomalous behavior of the viscosity, not yet accounted for in the theory.

Numercal Experiments of Humidity Change of Air Mass Moving from Seashore into Inland

In this paper are presented results of numerical study of the humidity change of air-mass, which moves from the sea into inland. Simultaneous differential equations (see Eqs. 1-3) as nonstationary two-dimensional diffusion of heat and water vapor in the lower atmosphere and of heat in the soil layer were numerically integrated by the difference method on an electronic computer. Profiles of turbulent diffusivity were estimated on the basis of KEYPS equation, in which is considered the effect of stability of lower air layer on the turbulent diffusivity of earch physical quantity. In numerical experiments soil moisture condition is parameterized by μ(=es/e(To)). The sets of physical parameters used in the experiments are presented in Table 1.The results obtained can be summarized as follows:1) Numerical experiments indicate that a mean K-profile calculated at the fetch of 7km can be used in investigating the air-mass transformation as representative one with acceptable error, although the turbulent diffusivity changes initialy some what rapid in both the profile and abosolute values, as air-mass moves over land (see Fig. 3).2) The height at which the difference of humidity (Δq=q(x, z)-q(o, z)) becomes 0, 1g/kg is adopted for characterizing (ZH) the thickness of internal boundary layer developing over the surface Fig. 7 shows the fetch dependence of boundary layer thickness. These relationships can be approximated by (ZH/L)=2.86×[(Kx/L2U)1/2]1.27, μ=0.4strong wind (ZH/L)=1.06×[(Kx/L2U)1/2]2.0, μ=0.25weak wind (ZH/L)=20.1×[(Kx/L2U)1/2]1.17, μ=0.6(ZH/L)=0.9×[(Kx/L2U)1/2]1.42, μ=0.4The thickness of internal boundry layer is thicker for weak wind conditions than for strong wind conditions. With increasing surface wetness, the humidity boundary layer becomes thicker due to intensive supply of water vapor from the ground surface.3) The part of diurnal changes of specific humidity, of temperature and of turbulent diffusivity are sensitive to the variation in thermal stability near the ground as shown in Fig. 10.Mean values of turbulent diffusivity (case-1) are greater for dry surface than for wet surface (case-5) from 1.1 to 1.5 times, indicating that increased thermal instability under dry surface conditions causes more strong turbulent mixing.4) Fig. 11 shows the variation in heat balance structure with the fetch. In the case of dry surface, total heat amount stored into layer during the daylight period was found to incease proportionally with the fetch.On the other hand, total heat amount transferred from the ground surface into air layer as sensible heat flux during the daylight period decreased somewhat, mainly because of decrease of temperature difference between air layer and the ground surface. It appeared that the heat balance structure at the wet ground surface did not very change with the fetch.

高電流・高速MIG溶接における溶融池現象に関する研究

The authors have studied the mechanism of formation, taking note of the discontinuous bead, particularly humping bead at high current and high speed welding. Following facts are clarified.1. In high current and high speed welding, the difference of temperature between the front part and the rear part of the molten pool is much greater than that in ordinary welding and this temperature difference causes strong surface-tension-stream directing from the front to the rear part of the pool resulting in the discontinuity of the bead.2. The authors have investigated some methods to prevent the formation of the discontinuous at high speed welding. The basic conceptions are as follows:a. Gravity control of the streamb. Decrease of temperature difference between the front and the rear part of the molten poolc. Decrease of surface tension of the molten poolThese methods aim to repress the strong stream from the front to the rear of the molten pool.For example, by elongating wire extension the arc takes the form of so-called rotating arc, which heats the molten pool uhconcentratively different from concentrative heating of spray arc and acts to prevent discontinuity of the bead. Fig. 5 shows the critical relation to get continuous at high speed welding. This curve is applicable to every diameter of wire.3. The authors analized (see equ. 4) of the and clarified the relation between efficiency and character. The results show that the welding speed at maximum melting efficiency is the limit to form the continuous bead.

アーク溶接継手の収縮過程に関する研究(第1報)

As a part of the research project on weld cracking of mild and high strength steels under hindered contraction, analytical attack was undertaken on the contraction process of weld joint during first-pass welding and cooling. Considerations on the heat flow in first-pass welding of heavy, steel plate show that contraction during cooling is proceeded by both weld metal contraction and decrease in thermal, expansion of mother plates. The latter is rather predominate. Contraction as the results of the decrease in thermal expansion of mother plate consists of two parts. The one is as the results of the decrease in temperature difference along thickness direction, and it is increased with increase of plate thickness (h). The other is as the results of overall heat loss from a part of the plate in a gauge length (l) over which contraction is measured. It is increased with decrease of heat capacity or thickness of plate, and is delayed with increase of gauge length even for a given thickness. As the results contraction is proceeded with different process as affected by thickness and gauge length. Some examples calculated for a usual arc-welding condition of mild steel are shown in Figs. 9, 10, 13 and 14.