Search

Small and imperfect crystals in polymers reorganize during slow heating. This leads to an increase of their melting point Tm. In order to measure the melting point of the original crystals, high heating rates are needed. This is possible with the modern heat-flow-calorimeters, which work with very small samples. The thermal lag of a DSC cell causes a shift of the melting peak by ΔT to higher temperature. From the theory of a heat-flow-calorimeter, it follows that the error ΔT is proportional to the square root of heating rate. heat of fusion and sample mass. Measurements with sharp melting low molecular weight compounds confirm that this square root relation is quantitatively followed. In order to measure the true melting point of the crystals present in a polymer sample, one has to use different high heating rates and constant sample mass. By plotting the melting peak temperatures as a function of the square root of heating rate and linear extrapolation to zero heating rate, the true melting point is found. This method is applied to HDPE, LDPE and some polyamides.

Long cans fabricated from alumina with various additives were found to be highly durable in the strongly reducing medium in carbon black reaction vessels and in the corrosive gaseous medium in glass-making furnaces. Good results were obtained also with cans protecting the thermocouples measuring the temperature of the hot blast of blast furnaces and the thermocouples used for studying the crystallization and cooling of steel ingots in their molds. Cans fabricated from stabilized zirconia were found to have satisfactory durability in nitrogen and argon media at 1900°C in induction furnaces with graphite heaters, and cans fabricated from magnesia gave satisfactory long-term performance in an argon medium at 2400°C. Cans fabricated from alumina, magnesia, and zirconia reduce the thermal lag of thermocouples owing to their high thermal conductivity and permit an extension of the range of temperatures measured in various media.

FromKing's linear equation a simple and universally valid expression for the sensitivity of hot-wires is derived.King's complete equation differs from the linear one by two correction terms, one of which is neglected as small. Using a result derived from the linear equation, the complete equation and its derivative are linearized. From this corrections are derived which are rather widely valid and which, when applied to the results obtained from the linear equation, lead to the more accurate ones corresponding toKing's complete equation. The measurements necessary to obtain the data for these corrections for a given kind of wire are easily made in still air without forced heat convection. The relation between pressure and temperature which in air influence the measurements without flow are explained on the basis of experimental results in connection with the similarity law for heat transfer by free convection. The curves given include the influence of the heating circuit upon the sensitivity and the thermal lag of the hot-wire.

Changes of length of a lead rod due to thermal expansion have been measured between 4.2° k and 10° k by a heterodyne-beat method in which the changes modify the capacity of a condenser in a resonant circuit. The variation with temperature of the Gfuneison constant y was deduced; the accuracy was just insufficient to show up the jump in thermal expansion coefficient at the superconducting transition.

This paper investigates the burning-rate response of a condensed-phase fuel plate adjacent to a reacting gaseous boundary layer, where the external flow has a longitudinal oscillation. The purpose is to simulate the unsteady droplet burning in a liquid-propellant rocket and to try to isolate a physical mechanism which leads to instability. The nonsteady process treated here is the heat transfer in the condensed phase. The results show that the velocity-sensitive responses exhibit resonant phenomena at the proper frequency range, and indicate that, in favorable conditions, a liquid-propellant rocket can be driven to instability because of the thermal lag in the droplets. Experiments to test this theory, and remedies to this type of instability, are suggested.

A brief history of solar stills in Chile is presented. Theoretical studies of solar stills in that country are described. These deal with efficiency, thermal balance, thermal intertia, thermal capacity, thermal conductance, thermal lag and distillation lag with regard to solar irradiation maximum, and time constant. Chilean solar still designs with evaporating trays of wood, metal, cement, and plastic, and evaporating cloths of various designs have been tested by the authors. Tests were made with glass and plastic covers in the Solar Energy Laboratory of the Universidad Santa Maria (Valparaiso) and in the Atacama Desert (Quillagua). Their purpose was to make theoretical predictions of still characteristics under different environmental conditions.

The coefficient of thermal expansion of a fluid can be measured by determining the critical temperature difference for the onset of thermal convection in a fluid layer heated from below. This method is particularly useful for fluids of very low thermal expansion because its accuracy is independent of the magnitude of the thermal expansion. It is easily adapted to measurement at elevated pressures. Accuracy of 0.5% is obtainable with care.

By use of the time-dependent diffusion equation it is shown that a heat-balance equation for an evaporating raindrop based on the “quasi-stationary” assumption (∂//t = 0) is accurate. This equation is then used to obtain the expression T = θw + zRdθw/dz for the steady-state temperature T of an evaporating drop in an atmosphere having linear vertical gradients of temperature and relative humidity, where θw is the equilibrium drop temperature in a uniform atmosphere (approximately the environmental wet-bulb temperature), dθw/dz is its vertical gradient, and zR is the thermal relaxation distance of the drop (falling at terminal velocity). The term zRdθw/dz is the thermal lag in the drop's response to the varying environment. Computations show that the ratio of the drop evaporation rate with the tag term to the rate without the tag departs significantly from unity for large drops and high humilities.

The problem discussed in this paper arose out of the need for accurately determining the temperature of a barometer under conditions of varying air temperature. A method for computing the thermal lag of simple and complex bodies of cylindrical form, when the ambient temperature is changing steadily, is given and formulae derived for determining the lagging times of such bodies in liquid and in gaseous media. Values calculated from the formulae are compared with results obtained experimentally.

In view of the complexity of the exact solution to the variational problem of constructing an optimum nozzle for gas-particle flow (see, for example, [1]) its solution in the one-dimensional approximation is of interest. In certain studies of this sort, by Marble [2] and Sternin [3], in addition to the assumption of one-dimensionality, an important role is also played by additional assumptions of smallness in the differences of the gas and particle velocities (and temperatures), which characterize the dynamic and thermal lag. The resulting linearization, used in [2, 3], not only narrows still further the region of validity of the one-dimensional approximation, but also leads to certain qualitative characteristics which do not correspond to the actual flow. For example, it is found [3] that variation of the contour curvature at only a single point may yield a finite thrust increment.

Since the results of the measurement of the X-ray-excited radioluminescence as a function of temperature (Part I of this paper) of TL dosimetry materials were difficult to interpret in terms of quenching of thermoluminescence, the direct measurement was made of the TL light sum and the glow-peak height as a function of heating rate. In order to overcome the problem of thermal lag between the heating planchet and the phosphor, a method was developed to achieve a high degree of thermal coupling by mixing powdered phosphor and gold and pressing the mixture to the surface of a gold disk. The glow curves from these gold plaques were recorded with a reader that produced heating programs which were linear to a few percent from rates of 4°C/min up to 640°C/min. By this technique glow-peak heights and light sums were measured for CaF2:Mn, natural fluorite, LiF(TLD-100), CaSO4:Mn, Li2B4O7:Mn, and terbium-activated lithium aluminosilicate glass over the full range of heating rates. Considerable variability was found in the thermal quenching characteristics of the various phosphors. All but the fluorite and the Li2B4O7:Mn showed some decrease in light output at high heating rates. LiF exhibited a decrease for low heating rates as well, with a broad maximum in the light output at about 100°C/min.

T USE of metallic fuel constituents in modern rocket engines has brought attention to nonequilibrium aspects of two-phase nozzle expansion processes. Since condensed metal oxide combustion products (which comprise 30 to 40% by weight of the total products of contemporary solid rockets) can do no expansion work, their presence in the rocket nozzle can only be deleterious to the effectiveness of the nozzle expansion process in converting thermal to kinetic energy. The condensed particles are accelerated in a nozzle almost exclusively by drag forces associated with lag or slippage of the particles relative to the expanding gas. Some performance loss relative to the calculated ideal no-slip expansion process must always be associated with macroscopic size particles, and experience has shown that the magnitude of the loss increases with the weight fraction of particles. Significant velocity and thermal lags thus have been suspected as a prime cause of rocket performance losses, and a number of studies have been directed toward delineation of the mechanism and magnitude of these lag effects. Early studies were summarized and extended in the review of Altman and Carter (l). Primarily, these early studies served to place bounds on the performance losses by examining the limiting cases of no-lag and complete lag. They demonstrated that thermal lag ordinarily has a lesser effect on specific impulse than does velocit}^ lag. Gilbert, Davis, and Altman (2) were the first to relate the losses to particle size. They solved the linear equation that results from assuming the drag force to be proportional to the velocity difference (Stokes' law) for the case of linearly accelerated nozzle gas. They demonstrated that, typically, a 1-ju diam particle follows the gas velocity closely, whereas a IQ-fj, diam particle has a significant lag. All of these early studies treated the nozzle expansion processes as though they are uncoupled, i.e., the thermal lag

Amorphous polymers thermal conductivity measured at 0.4-4 K, observing similar temperature dependences

A combined finger-plethysmograph and gradient calorimeter has been devised, especially to study cold-induced vasodilation in the finger. By using an electrical heater and a flow-heater it was shown that the thermal lag depends on thermal capacities, conductivities and temperature distribution, so that heat loss and flow are correlated only in the steady state. The actual drop of blood temperature from inflow to outflow was calculated. Results were: a) skin temperature was a better estimate of outflow than bath temperature, b) with general vasodilation, the inflow temperature was close to deep body temperature, but c) with vasoconstriction, digital arterial temperature might be as low as 14°C, and the calculated minimum blood flow (Greenfield) 70% too low. Plethysmography was possible during the hunting reaction in spite of a 30% decrease in venous distensibility. When the subject was cold, blood flow and heat loss, both in constriction and dilation phases, were reduced two or three times, probably due to the increased viscosity of blood. Submitted on August 3, 1959

In the previous part of the paper (1)' we discussed the fundamentals of cooling of a hot-wire and proposed a methed to compensate its thermal lag. Then in this paper, taking foundation on these theoretical results, we will show several methods to measure turbulent velocity fluctuations. We use the same symbols as in the previous paper (1) .

An experimental investigation of the characteristics of thermistors, when used as anemometers, has been made. The various modes of operating thermistor micro-anemometers are studied, and the sensitivity and errors discussed. A variable-temperature, variable-speed wind tunnel was constructed for this investigation, to determine the power dissipated in the thermistor as a function of wind speed, wind direction, and air temperature. The thermal lag of the thermistor to environmental changes was studied. It was concluded that operating the thermistor at constant resistance, by means of a specially designed bridge, showed the greatest promise in that this mode of operation was the most sensitive to wind-speed changes, the least sensitive to ambient-temperature changes, and eliminated lag to a great extent. Operated at constant resistance, the lag time of the response of the thermistor to a “step-function” air-speed change was about 0.085 sec, the error in air-speed indication due to a degree change in...

IN MISSILE applications, electric equipment is required to function for a relatively short period of time in a hot ambient atmosphere, but is deprived of external cooling media such as air or oil. The dependability of the equipment depends on its ability to absorb its own heat losses or to transmit these losses by conduction to cooler surroundings. The general purpose of this paper is to present a method for predicting simultaneously the transient temperatures in two heat-generating solids such that both materials store some heat, while some heat is transmitted from one to the other and thence possibly to a third region at a fixed temperature. In particular, mean transient temperatures are predicted in the field winding and laminations of an a-c generator which is bolted to a support, but otherwise insulated from the ambient atmosphere. Application of this method to temperature prediction in solid-state devices is natural.

A two-velocity and two-temperature model is considered for a continuous medium in relation to the flow of a mixture of gas and particles in the subsonic, transsonic, and supersonic parts of a Laval nozzle. It is assumed that the particles are small, and hence that the coefficients ϕf and ϕq, which define the interaction with the gas, are large (these coefficients are inversely proportional to the square of the particle radius for a Stokes mode of flow). This means that the velocity or thermal lag of the particles relative to the gas is small. The solution is sought as expansions with respect to the small parameters ɛ1=1/ϕf and ɛ2=1/ϕq.

The theory of programmed temperature gas chromatography, particularly in relationship to the spreading of peaks as measured by the plate height is expanded. The significance of the plate height in programmed temperature operations is examined. The close relationship of this with - the plate height in isothermal chromatography is discussed. Two phenomena which are unique to the programmed methods are treated theoretically. First is the zone compression at the head of the column which is a result of temperature being initially low followed by continual heating. Second is the effect of thermal lag which is responsible for a higher zone velocity at the wall than at the center of the column. This effect is treated in terms of the nonequilibrium theory which was developed for various plate height calculations in chromatography.

A method for measuring the thermal characteristics of natural rocks at low temperature is developed. The advantage of the experimental technique is that both thermal conductivity and thermal diffusivity are obtained from the same run by interpreting two parts of the temperature profiles. The quasi-steady state is used for thermal conductivity, and the thermal diffusivity is obtained from the asymptotic transient state (Fourier transient state).

Kinetics is of fundamental importance in the understanding of the mechanism of thermal decomposition of high polymers. Since most thermal decomposition reactions are associated with weight changes, thermogravimetry is being employed to a great degree in kinetic studies of polymer decomposition. Two approaches are commonly used to obtain the various kinetic Chapaumeters of these reactions. The static method or “isothermal thermogravimetry” produces precise weight loss curves for various single temperatures as a function of time. Thus the rate Chapaumeters are obtained directly. This method is inherently time consuming and the results are sometimes questionable due to experimental uncertainities arising from the initial thermal lag of the sample. More recently, “dynamic thermogravimetry” data has been used for kinetic studies. In these, a single weight loss curve obtained by means of programmed heating is used to provide information equivalent to an entire family of isothermal weight loss curves. The new approach is time saving, being able to cover a wide range of temperatures in a relatively short time. For this work the kinetics evaluation procedure by Freeman and Carroll1 and its extension by Anderson and Freeman2, is applied to dynamic thermogravimetric data in order to evaluate the rate Chapaumeters.

REPTILES reportedly possess physiological mechanisms for the increase or decrease in the rate of heat exchange with their environments. This the animal could do by changing either its metabolic rate or its effective insulation. Both sorts of mechanisms have been postulated responsible for this capability.

A constant temperature hot-wire anemometer can be used to compensate for the thermal lag of a wire or film resistance thermometer within the useful frequency range of the anemometer. This method may be used as an alternative to the standard method in which a differentiation circuit as employed in constant current anemometers is used for frequency compensation. The new method is useful if a constant temperature anemometer is at hand. The time constant of the resistance thermometer need not be determined.

IN some recent investigations on adsorption1 it seems to have been established that erroneous conclusions have been reached on account of errors unsuspected by the experimenters and arising from the time lag of thermometric apparatus. Workers with experience in thermal measurements are well aware of the possibility of such errors, and in particular the extreme care which must be exercised in the use of platinum resistance thermometers, more particularly when these are wound on insulating supports and enclosed in tubes. The application of such thermometers requires not only extreme care in avoiding thermometric forces in the circuit, but also in making certain that the very appreciable thermal lag is eliminated. Generally speaking, such thermometers are quite unsuitable for use when fairly rapid changes of temperature are involved, and in any case are best avoided, except by experienced workers. For practically all purposes, a properly chosen mercury thermometer is much more trustworthy.

O design of air-breathing propulsion systems will require an adequate description of the mixing and combustion of ducted, fuel-rich, primary streams with secondary air. The primary stream will often contain metal particles such as boron, magnesium or aluminum. Development of a model has been previously reported' for describing combustion of a particleladen jet which is mixing with a subsonic secondary air stream in a duct. This model considered the particles to be in dynamic and thermal equilibrium. This synoptic summarizes an extension of this earlier model to include particle nonequilibrium effects. Particle drag, thermal lag, and particle ignition, melting, and combustion effects are considered. A series of parametric calculations for a typical boron-loaded, air-augmented system shows effects of particle size and secondary duct pressure on metal combustion efficiency. Methods of improving combustion efficiency at low pressure are briefly examined.

MANY workers have attempted to observe metal/mould interface temperatures in metal castings by means of thermocouples with sheathed hot-junctions. However, since the temperature gradient in the refractory sheath is high, such measurements are liable to considerable error. We have therefore made measurements on an experimental steel casting with an unsheathed platinum / platinum - 13 per cent rhodium hot junction, the metal of the casting being used as intermediate metal; the two wires were simply pushed through the mould wall some 0.3 cm. apart. The technique has the following advantages: (i) it eliminates errors due to the temperature gradient across the sheath; (ii) it minimizes thermal lag; (iii) it ensures accurate location of the hot junction at the metal/mould interface, since the hot junction is formed at the points where the molten metal comes into contact with the thermocouple wires.