Reduction In Temperature Gradient Research Articles

Radio Occultation Exploration of Mars

The radio occultation technique, consisting of the observation of changes in the phase, frequency, and amplitude of a radio signal from a spacecraft as it passes through the atmosphere of a planet before and after occultation, was first applied to measure the atmosphere of Mars with the Mariner IV spacecraft in 1965. The interpretation of these changes in terms of refraction of the radio beam by the neutral atmosphere and ionosphere of the planet provided the first direct and quantitative measurement of its vertical structure and established the surface atmospheric pressure of Mars as lying between 5 and 9 mb. The presence of a daytime ionosphere with a peak electron density of about 105 el cm−3 was also measured. The Mariner VI and VII spacecraft flew by Mars in 1969 and provided an additional four measurements of the atmosphere and surface radius of the planet. They confirmed the surface pressure values measured by Mariner IV and provided data for a crude estimate of the shape of the planet.By far the greatest volume of radio occultation information on the atmosphere and surface of Mars was returned by the Mariner IX orbiter which was placed in orbit about Mars in November of 1971. During three occultation episodes in November-December 1971, May-June 1972, and September-October 1972, the Mariner IX mission provided 260 successful radio occultation measurements.The early measurements, made at the time of the Martian dust storm of 1971, showed greatly reduced temperature gradients in the daytime troposphere, indicating the heating effect of the dust. The temperature gradients that were measured later in the mission, when the atmosphere was apparently free of dust, were still much lower than expected under conditions of radiative-convective balance, indicating that dynamics may play a large part in determining the temperature structure of the Martian troposphere. Temperatures taken at night near the winter poles were consistent with the condensation of carbon dioxide.The surface atmospheric pressure was observed to vary widely with topography ranging from about 1 mb at the summit of the Middle Spot volcano (Pavonis Mons) to over 10 mb in the North circumpolar region. In the South equatorial region the highest surface pressure of about 9 mb was measured at the bottom of the Hellas basin.The radius of the planet was measured with accuracies ranging from about 0.25 to about 2.1 km over latitudes ranging from 86° to −80°. These measurements have shown that Mars has pronounced equatorial and north-south asymmetries, which make it difficult to represent its shape by a simple triaxial figure.The daytime ionosphere measurements indicated that the main ionization peak was similar in behavior to a terrestrial F1 layer and is probably produced by photoionization of carbon dioxide by solar extreme ultraviolet. Comparison of the heights of the maximum between the early data taken in November-December, 1971, and the Extended Mission of May-June 1972, showed that the lower atmospheric temperatures decreased by about 25%, which is consistent with clearing of the atmosphere.The experience gained from Mars radio occultation experiments suggests that the quality of data can be significantly improved by such features of the spacecraft radio system as a stable oscillator, dual frequency downlink capability, and a steerable high-gain antenna.

A Device for the Removal of Temperature Gradients in NMR Spinning Tubes

A simple paddle device is described for the removal of temperature gradients in liquids contained in high resolution NMR spinning tubes. Experiments show that this accessory reduces temperature gradients to less than 0.10°C along the length of the liquid without any impairment in spectrometer resolution or sensitivity. The device is readily adaptable to most conventional high resolution NMR spectrometers.

The vapour growth of IV–VI compounds

The various techniques for the vapour growth of both bulk crystals and thin films are briefly described with especial reference to those which have been used in the growth of IV–VI compounds (PbTe etc.). The special problems which arise in the vapour growth of solid solutions between pairs of IV–VI compounds - materials of interest and importance in the IR detector and laser fields - are discussed. As a result of experience in growing Pb 1- x Sn x Te epitaxial layers, a novel and effective vertical epitaxial system, incorporating a thermal block to reduce temperature gradients and fluctuations in the substrate(s) and continuous rotation of the substrate holder in a plane perpendicular to the gas flow, has been designed and constructed. This system has proved very succesful in the growth of Pb 1- x Sn x Te by sublimation and would lend itself to the growth of other compounds both by sublimation and by chemical transport.

The management of infants for cardiac surgery under deep hypothermia.

Deep hypothermia is now employed for open heart surgery in infants. The method of surface cooling infants to 25° C and then by-pass cooling down to 15–20° C prior to exsanguination is described. This provides a still heart and ideal operating conditions. The infants are re-warmed on by-pass. Some of the physiological effects of hypothermia and the methods employed to counteract these are discussed. The aims are to reduce temperature gradients, improve cerebral oxygen supply, avoid arrhythmias and secure haemostasis. The experience in 33 operations is outlined. Seventeen patients were under one year of age.

The atmosphere of Mars from mariner 9 radio occultation measurements

The Mariner 9 spacecraft was used to perform 160 radio occultation measurements in orbit about Mars during November and December of 1971. At that time, Mars was experiencing a severely obscuring global dust storm. The effect of dust in the atmosphere was reflected in the reduced temperature gradients that were measured in the daytime near-equatorial atmosphere, indicating heating of the atmosphere by solar radiation being absorbed by dust and a simultaneous cooling of the surface. The temperature gradients increased somewhat with time, possibly indicating a gradual clearing of the atmosphere. Measurements made at 65° latitude near the morning terminator showed atmospheric temperatures consistent with condensation of carbon dioxide at low altitude.The surface pressures in the near equatorial regions ranged from a high of 8.9 mbar in Hellas to a low of 2.8 mbar in the Claritas and Tharsis areas, with a mean pressure of 4.95 mbar. The pressures deduced from measurements at 65° latitude ranged from 7.2 to 10.3 mbar, with a mean of 8.9 mbar.The pressure altitudes, referred to a pressure level of 6.1 mbar, show a range in the equatorial regions from a low of −4.4 km in Hellas to a high of 9.6 km in Claritas, with a net excursion of 14.0 km and a mean altitude of 2.7 km. In contrast, the region at 65° longitude shows uniformly negative altitudes, with a mean of −2.6 km. This disparity in pressures, which is also reflected in the measured radii between the near-equatorial and 65° latitude measurements, strongly suggests that the physical shape of Mars is more oblate than the shape of its gravitational equipotential surface, leading to higher atmospheric pressures near the poles than at the equator. However, more measurements at high latitudes are necessary to support this hypothesis.A daytime ionosphere having a peak density of about 1.5 − 1.7 × 105el/cm3 at an altitude of 140-134 km over a range of solar zenith angles of 56-47° was measured, showing some correlation between the variations in the peak density and the solar flux measured from the earth. The average topside plasma scale height was 38.5 km, showing little correlation with solar flux and solar zenith angle.

Heat transfer during solidification of layered intrusions. I. Sheets and sills

With the advent of crystal settling in a solidifying igneous intrusion, the roof contact of the intrusion may be continuously subjected to temperatures approaching those of the magma itself. This causes steeper temperature gradients in the roof rocks and, consequently, more rapid heat loss and solidification than would occur if the crystals were frozen to the roof. By contrast, the accumulation of crystals in the lower parts of the intrusion acts to lower the floor contact temperature and so reduces temperature gradients in the floor rocks; but since the proportion of liquid near the floor is reduced, the overall effect also is to speed solidification, specifically in this case, from the floor upwards. The present paper reviews in a qualitative way the relation between crystallization mechanisms and temperature in a magma body when crystal fractionation is a major process. It then examines quantitatively, by means of one-dimensional solutions of the heat flow equation, the influence on crystal accumulation rates and roof rock temperatures of (a) variations in magma temperature, (b) pre-intrusion temperature gradients in the roof rocks, (c) shallow emplacement, (d) formation of an upper border zone or floating of crystals, and (e) latent heat effects associated with contact metamorphism. Finally, it considers the effects of heat loss to the floor rocks on (i) crystallization in the magma above the accumulating pile of crystals, (ii) solidification of trapped pore magma, and (iii) contact metamorphism of the floor rocks. Some of the more important equations presented also apply to sills, sheets, and dikes showing no crystal fractionation.

Use of a sodium jacket for temperature equalization

A device for simultaneously equalizing temperatures and greatly reducing temperature gradients in test furnaces has been developed. This device consists of a liquid-sodium filled furnace liner which eliminates the need for multiple zone furnaces for most applications.

Alleviation of Thermal Stresses in Bi‐metallic Spar Webs

Results of an Investigation to Determine the Effectiveness of a Bi‐metallic Construction on Four Three‐spar Box Specimens Subjected to Simulated Aerodynamic Heating. The maximum thermal stresses in a stainless steel skin‐web combination subjected to aerodynamic heating may be greatly reduced by the addition of a thin layer of aluminium or copper to the stainless steel web. Such bi‐metallic construction increases the rate of conduction of heat from the structural parts (skins) exposed to the aerodynamic heating to the non‐exposed parts (webs) and consequently this produces a reduction in temperature gradients and thermal stresses. This paper presents the results of an investigation to determine the effectiveness of a bi‐metallic construction on a three‐spar box specimen subjected to simulated aerodynamic heating. The tests confirmed the feasibility of the method and indicated that the theoretical calculations give sufficient accuracy for design studies, provided an estimate of the joint conductance can be made. The method is effective only for relatively low heating rates and shallow wing structures. The work reported in this paper was sponsored by the Aerospace Research Laboratories, Wright‐Patterson Air Force Base, Ohio, under project No. 7063, Task 706302.

The cooling of electrical machines

This paper is intended to serve the dual purpose of recording the present-day attitude towards cooling electrical machines and of describing tests carried out to fill the gaps left by the hitherto published data.In a simple air-cooled machine the temperature-rise of the hot spot is built up in three stages' —θt = θg + θs + θmwhere θt = hot-spot temperature-nse,θg = temperature-difference between the hot spot and the dissipating surfaces over which the air flows,θs = temperature-difference between dissipating surface and cooling air, andθm = temperature-rise of cooling airIt is believed that this three-term equation is the key to rational design of machines in regard to their cooling, and in the paper the subject is studied on a basis of this equation. Although some engineers now deal with temperature problems on these lines, the older practices, of which an example is the rating of field coils according to the watts dissipated per square inch of surface, are still the standard in many quarters The first part of the paper describes the limitations met with when approaching the problem of cooling. The standard permissible temperatures in Great Britain, Germany and America arc examined, and the restrictions due to various types of enclosure are shown.The merits and disadvantages of radial, axial, and other forms of ventilation are then discussed in relation to the different classes of machines now being manufactured, and the broader problems of cooling are reviewed. Following this, the subject is gone into in detail, and the internal flow of heat in the different classes of apparatus is investigated. The results of various experiments to improve the heat flow by reducing temperature gradients are given. The ability of various types of ducts and cooling surfaces to transfer heat to air is discussed, and comments on predetermining the temperature-rise of a machine are included. The flow of air through the various paths met with in electrical apparatus is also investigated, and the results of certain experiments on this subject arc tabulated.The design of fans for electrical machines is next considered, and the special features introduced by the combination of the fan and the electrical machine are dealt with. The paper concludes with some notes on the progress in cooling during the last 30 years, and summarizes the present-day position.