Showing papers on "Latent heat published in 1983"

Thermodynamic Properties of Steam in the Critical Region

Abstract: An analysis is presented of the experimental data on thermodynamic properties in the critical region of steam. The model used is that of revised and extended scaling, as given by the modern theory of critical phenomena. All thermodynamic properties are given in closed (parametric) form. The model has, in addition to three universal constants that are given by theory, sixteen adjustable parameters that were obtained by least‐squares fit to PVT and speed‐of‐sound data. It is valid in the range 200–420 kg/m3 in density and 644–693 K in temperature. It accurately represents the experimental data for equation of state, vapor pressure, latent heat, specific heats Cp and Cv and speed of sound. Our analysis permits new estimates of the critical parameters of steam, and has led to a number of conclusions regarding the mutual consistency of the experimental data. Tabulated values of the thermodynamic properties of steam are appended to the paper.

On the Use of Bulk Aerodynamic Formulae Over Melting Snow

Abstract: Bulk aerodynamic formulae which relate the turbulent exchanges of sensible and latent heat over melting snow to measurements of windspeed, temperature and humidity at one level can be derived from flux-gradient relationships and assumed log-linear profiles Recent analyses of local advection over snow and wind flow over complex terrain suggest that the bulk aerodynamic formulae should apply in non-ideal field situations The assumption that the scaling lengths for temperature and humidity equal the roughness length is problematic, since theoretical analyses indicate they should be much less than the roughness length However, the effect of scale length inequality on the stability correction tends to compensate for the effect on the neutral-case transfer coefficient Field experience indicates that the bulk aerodynamic formulae are adequate for use in energy balance estimates of daily or shorter term snowmelt

Numerical Simulation of the Airflow over Lake Michigan for a Major Lake-Effect Snow Event

Abstract: A mesoscale model is used to simulate the airflow over Lake Michigan for the major lake-effect snowstorm of 10 December 1977. This storm was characterized by a land breeze circulation and a narrow shore-parallel radar reflectivity band. The model successfully simulated the major atmospheric circulation features including a mesoscale low pressure center and a land breeze front. The model also captured the general character of the observed precipitation pattern which was typified by a narrow band of heavy precipitation along the eastern shore of Lake Michigan. Further simulations were made to examine the effects of latent heat release, lake surface temperature distribution and model grid resolution upon the simulation. Latent heat release was found to have an important effect in strengthening convection. However, the basic land-breeze circulation was found to develop for the simulated conditions even without latent beating. For a given mean lake-land temperature difference, details of the lake surf...

Heat Capacity and Enthalpy Behavior Near Phase Transitions in Some Alkylcyanobiphenyls

Abstract: An adiabatic scanning calorimeter has been used in a calorimetric investigation of alkyl-cyanobiphenyl (nCB) liquid crystals. Measurements near solid to smectic (KA), smectic A to nematic (AN) and nematic to isotropic (NI) or smectic A to isotropic (AI) phase transitions have been carried out for scanning rates as low as a few μKs−1. Results for the temperature dependence of the heat capacity as well as for the enthalpy (including latent heats) have been obtained for 8CB, 9CB, 10CB and 11CB. Latent heat values are reported for the different KA, NI and AI transitions. For the two compounds 8CB and 9CB, which show a AN transition, it was found that, within the resolution of the experiments, these transitions are second-order. The anomalous heat capacity observed near the AN transition is much larger for 9CB than for 8CB. The critical exponent α obtained from power law fits to the heat capacity data is 0.31 ± 0.03 for 8CB and 0.50 ± 0.05 for 9CB. It is suggested that the AN transition in 9CB must be...

Experimental Studies of the Turbulence Structure Parameters of the Convective Boundary Layer.

Abstract: We present a collection of experimental data concerning turbulent characteristics of the convective boundary layer. These data were obtained by means of instrumented aircraft during numerous experiments carried out above five flight areas from 1973 to 1978. Only dry convective conditions with weak dynamic instability are contained in this report. The computed quantities are vertical turbulent fluxes of sensible and latent heat, second- and third-order moments of vertical wind component, potential temperature and humidity, dissipative and spectral characteristics and length scales. All these quantities are normalized using the convective similarity hypothesis of Deardorff. A particular case is considered for humidity where it is necessary to choose between the two boundary conditions: evaporation and entrainment flux. The latter is chosen.

A study of heat and moisture budget over the Arabian Sea and their role in the onset and maintenance of summer monsoon

Abstract: The influence of heat, moisture and moist static energy (MSE) budget, over the Arabian Sea and adjoining area (0°-30°N and 30°E-75°E), on the onset and activities of Asian summer monsoon has been studied in detail. The data base for this study consists of twice daily FGGE Level IIIb analysis for the period 16 May to 15 July 1979. The pentad mean variation, the vertical distribution and period averages of the various terms in energy budget equations are closely examined to find out their influence on the activities of monsoon. The study indicates the significant increase in the net enthalpy, latent heat energy (LHE), MSE and a number of budget parameters, well in advance of the onset of monsoon over Kerala coast. Further, a decreasing trend is observed in most of the above parameters about 5 days before the break monsoon condition which started over India on 16 July, 1979. The vertical distributions of the budget parameters reveal that during active monsoon period secondary maxima of horizontal heat and MSE flux divergences are observed in the upper troposphere which are replaced by minima during weak monsoon circulation. The broad features of the budget studies over the Arabian Sea are in good agreement with the large scale energetics (Mohanty et al. 1982a, 1982b). Some of the significant departures in the results of the two studies have been discussed.

Freezing in a Vertical Tube

Abstract: Fundamental heat transfer experiments were performed for freezing of an initially superheated or nonsuperheated liquid in a cooled vertical tube. Measurements were made which yielded information about the freezing front and the frozen mass, about the various energy components extracted from the tube, and about the decay of the initial liquid superheat. Four component energies were identified and evaluated from the experimental data, including the latent energy released by the phase change and sensibly energies released from the subcooled frozen solid and the superheated liquid. Initial superheating of the liquid tended to moderately diminish the frozen mass and latent energy extraction at short freezing times but had little effect on these quantitites at longer times. The extracted sensible energies associated with the superheating more than compensated for the aforementioned decrease in the latent energy. Although the latent energy is the largest contributor to the total extracted energy, the aggregate sensible energies can make a significant contribution, especially at large tube wall subcooling, large initial liquid superheating, and short freezing time. Natural convection effects in the superheated liquid were modest and were confined to short freezing times.

An investigation of a polynya in the Canadian Archipelago: 3. Surface heat flux

Abstract: The heat loss from the Dundas Island polynya is calculated for a 3 week period during March 1980. Bulk and free convection formulae based on sonic anemometer-thermometer measurements are used to calculate the sensible and latent heat fluxes. These terms, along with simple models to calculate the albedo and the long-wave radiation exchanges, yield a surface heat budget for the polynya. The sensible heat flux for the period March 12, 1980, to April 1, 1980, averaged 204 W/m2 and the mean total heat loss from the polynya was 329 W/m2.

Transfer processes at the air-sea interface

Abstract: Near-surface data from ships, buoys, aircraft and a microwave remote-sensing satellite have been used to estimate the fluxes of momentum, heat and water vapour at the sea surface over a 200 km x 200 km area during the Joint Air-Sea Interaction Experiment of 1978. In particular, daily means of the surface heat balance and the wind stress are presented. Generally, the sensible heat flux was found to be less than 25 % of the latent heat flux. Over periods of a day the total upward heat flux was about a third of the net radiation, implying that a significant proportion of the available energy went into heating the ocean. The Ekman pumping accounted for most of the divergence in the atmospheric boundary layer but only 10 % at most of that in the upper ocean. Some case studies of the horizontal variation of the fluxes in relation to larger scales are also discussed and it is suggested that the fluxes are modulated by mesoscale patterns in sea-surface temperature.

The Impact of Model Moist Processes on the Energetics of Extratropical Cyclones

Abstract: A diagnostic energetics analysis is used to study the effects of moisture-related processes on two numerically simulated cases of extratropical cyclone development Model output is derived from moist and dry versions of the Drexel University Limited Area Mesoscale Prediction System (LAMPS) Both kinetic and available potential energy budgets are partitioned into zonal and eddy quantities in order to focus more effectively on cyclone-scale processes Results from the LAMPS forecasts show that the baroclinic aspects of cyclone development are significantly enhanced by both convective and stable latent heat release Although generation of available potential energy (GE) is an important source of eddy available potential energy (AE), diabatic heating produces substantial modifications to eddy energy conversion terms Transformation of zonal available potential energy (AZ) to AE is strengthened in the moist forecasts through increased horizontal eddy transport of sensible heat Although conversion of

Modeling the diurnal dependence of the optical refractive index structure parameter

Abstract: A model which simulates the diurnal dependence of the optical refractive index structure parameter within the atmospheric boundary layer is constructed by solving an energy balance equation for the ground surface temperature. This solution yields surface fluxes of sensible and latent heat. Empirical relationships are then used to calculate Cn2. Model results compare favorably with data obtained at a desert location under clear skies. A sensitivity analysis indicates that Cn2 is particularly sensitive to time of year, wind speed, the product of soil heat capacity and soil density, and soil moisture content. This model is applicable over relatively flat surfaces with little or no vegetation.

Thermochemical energy storage

Abstract: A thermal energy storage medium comprising a hydrophilic polysaccharide supporting an inorganic salt that is capable of transforming from one phase to a less hydrated phase absorbing latent heat, and releasing this latent heat upon the reverse transformation. The polysaccharide, preferably Xanthan Gum, may be incorporated in concentrations of 0.05% to 3% together with a nucleating agent in order to form a material that transforms when cooled back to the transformation temperature, or in greater concentrations of 1% to 5% without a nucleating agent to form a material that may be cooled below the transformation temperature without transformation taking place, and stored at ambient temperature while still storing the latent heat until activated. The medium is gelled in the less hydrated phase and in some embodiments the gel is pseudoplastic thus enabling it to be poured into chambers of an energy storage device, and then regain its original viscosity at rest.

Dynamics of latent heat storage in fixed beds. A non linear equilibrium model. The analogy with chromatography

Abstract: This paper investigates theoretically the dynamics of a latent heat storage system consisting of a fixed bed operating close to thermal equilibrium with a percolating fluid. The model used is an idealized one, which emphasizes the role of the heat capacity factor, but neglects heat transfer resistances, losses, and hydrodynamic non-idealities. The approach is shown to resemble that used classicaly in non-linear adsorption, and analogous concepts, such as shock waves and dispersive waves are introduced. The shapes and propagation characteristics of the temperature profiles generated during loading and unloading of the bed are discussed, for various fusible materials, operating temperatures, initial and boundary conditions, and flow direction. The non-linear dependence of the enthalpy of the storage material on temperature is shown to play a central role in determining the qualitative features of these profiles. The aims of the article are mainly qualitative, conceptual and didactic.

Influence of Water Stress on the Diurnal Exchange of Mass and Energy between the Atmosphere and a Soybean Canopy1

Abstract: A micrometeorological-physiological study was conducted at Mead, Nebr., during the summer of 1980 to examine the diurnal exchanges of mass and energy of well-watered and water-stressed soybean (Glycine max (L.) Merr. cv. Harosoy) canopies and to relate these exchanges to environmental and physiological variables. Data are presented for 2 clear days when the canopy was fully-developed. Measurements of CO2, latent heat and sensible heat flux were made using the Bowen-ratio energy balance technique. The soil of the area is a Sharpsburg silty clay loam (a fine, montmorillonitic, mesic Typic Argiudoll). Water stress greatly influenced the partitioning of available energy between latent and sensible heat flux. When the crop was well-watered, sensible heat (H) was directed toward the crop and caused latent heat exchange (LE) to exceed net radiation (Rn). When the crop was water stressed, only two-thirds of Rn was consumed as LE; the remainder was converted into sensible heat. Since both Rn and vapor pressure deficit were greater on the day when the crop was water-stressed, stomatal closure appears to have been the primary cause of the reduction in LE. Carbon dioxide exchange was not sensitive to water stress in the morning but was severely limited by such stress during midday. The midday reduction in CO2 exchange appears to have been caused by a combination of high stomatal resistance limiting CO2 diffusion to the cell chloroplasts and low leaf water potential coupled with high air temperature affecting the enzymatic reactions associated with photosynthesis. Water use efficiency (defined in terms of the CO2-water flux ratio) was greater when the crop was well-watered than when it was stressed for water. A combination of water stress, a large vapor pressure deficit, and high air temperature reduced the CO2-water flux ratio. Additional index words: Glycine max (L.) Merr., Micrometeorology, Water use efficiency, Photosynthesis, Evapotranspiration, Canopy CO2 exchange, Environmental physiology. E and physiological variables affect mass and energy exchanges between a crop canopy and the atmosphere in a mutualistic, synergistic, or antagonistic manner. Micrometeorological measurements, supported with physiological data, provide one of the best means of examining these complex processes. The ad1 Published as Paper No. 6779, Journal Series, Nebraska Agric. Exp. Stn. The work reported here was conducted under Regional Research Project 11-33 and Nebraska Agric. Exp. Stn. Project 11-49. Received 28 Jan. 1982. 2 Formerly research associate (now postdoctoral fellow at Atmospheric Turbulence and Diffusion Laboratory, P. O. Box E, Oak Ridge, TN 37830), associate professor, professor, professor and visiting scientist, Center for Agricultural Meteorology and Climatology, and associate professor, Dep. of Agronomy, Institute of Agriculture and Natural Resources, Univ. of Nebraska, Lincoln, NE 68583-0728. vantage of using micrometeorological techniques is the ability they afford to measure spatially integrated exchange rates on a continuous basis, without influencing the local microclimate. Over the past 20 years, most comprehensive micrometeorological-physiological studies have been performed over crops in a well-watered condition—corn, wheat, alfalfa, and sugar beets (e.g., Lemon, 1960; Monteith and Szeicz, 1960; Denmead, 1969; Saugier, 1970; Brown and Rosenberg, 1971; Baldocchi et al., 1981a, 1981b). Only a few such studies have been attempted with water-stressed crops (Begg et al., 1964; Biscoe et al., 1975). There have also been very few detailed studies of the mass and energy exchanges between the atmosphere and soybeans, an economically important crop. Bailey and Davies (1981) is an exception to this statement. The objective of the study reported here is to examine the diurnal exchanges of mass and energy of well-watered and water-stressed soybean canopies. The influence of environmental and physiological variables on these exchanges are discussed in detail. Results are compared with findings derived from chamber studies on soybeans. MATERIALS AND METHODS Experimental Details The study was conducted during the summer of 1980 at the Univ. of Nebraska Agricultural Meteorology Laboratory at Mead, Nebr. (41° 09' N; 96° 30' W; alt. 354 m above msl). Soybeans (Glycine max L. Merr. cv. Harosoy) were planted on 22 May 1980, in an experimental field (65 m E-W by 210 m N-S) in 0.75 m wide north-south rows. Border fields to the east, south, and west were also planted in Harosoy cv. soybeans. Data used in this study were selected from periods when the fetch to height ratio exceeded 75 to 1. Between 20 July and 10 August the canopy was erect but was water-stressed. A storm on 10 August relieved the water stress but caused lodging in the crop. Air temperature and vapor pressure were measured over the field at 1.25, 1.50, 1.75, 2.25, 2.75, and 3.25 m with an automatic, self-checking, multilevel psychrometer (Rosenberg and Brown, 1974). Once each hour the psychrometer assembly rotated automatically into a horizontal position for calibration. Air was sampled to determine CO2 concentration with a multilevel manifold at 0.30, 0.50, 0.70, and 0.90 m within the crop 544 AGRONOMY JOURNAL, VOL. 75, MAY-JUNE 1983

Heat fluxes over the eastern tropical Pacific and aspects of the 1972 El Niño

Abstract: Recently evaluated flux formulas were used with an edited ship-of-opportunity data set for the 1970's decade to derive estimates of the individual surface heat fluxes and the total or net surface energy exchange for the eastern tropical Pacific Absorbed solar radiation is the dominant heat flux in the region, and latent heat exchange is the second most important component; total cloud cover and wind speed are generally the most significant atmospheric variables Over much of the area, the annual excess of net surface flux appears to be balanced by heat advection in the South Equatorial Current Although the seasonal cycles of sea surface temperature seem to be produced by variations in net surface flux over parts of the area, they do not appear to balance off Peru and near the equator Instead, water temperature seems to be noticeably affected by the seasonal cycle of upwelling and advection along the Peru coast This data set was also used to derive sea surface temperature and net surface heat flux departures from the mean for individual months during the 1972 El Nino The magnitude and speed of the initial warming along the eastern ocean margin indicate changes in heat content at least an order of magnitude greater than any departures of net surface flux; this fact and the subsequent poleward and westward spreading of warm water support an advective origin, especially Wyrtki's hypothesis of an eastward propagating Kelvin wave Subsequent changes during an El Nino episode also do not seem to be influenced appreciably by variations in surface heat exchange

A comparison of the snowmelt energy budgets in two alpine basins

Abstract: The snow surface energy budgets in two alpine basins in New Zealand's South Island were calculated from meteorological observations made during a spring melt period. The sensible heat flux was the most important energy source and precipitation heat flow the least important at both sites. Latent heat was the second greatest source at the more maritime site while net radiation was more important than latent heat at the other site. The greater cloud cover at the more maritime site accounts for the lower net radiation received there. The differences in the sensible and latent heat fluxes are due to differences in the air temperature, humidity and wind speed between the sites. No reliable relationship between the wind speeds at the two sites appears to exist. The differences in temperature and humidity result from the influences of different airmasses and the modification of airmasses as they traverse the mountain ranges.

Latent heat storage and supply system and method

Abstract: Latent heat storage and supply system and method in which a body of crystallizable phase change material and a separate body of a special crystallization nucleator for the phase change material are associated for heating as a unit to melt the phase change material, for cooling to bring the molten phase change material to supercooled condition in which heat is stored as latent heat of crystallization and thereafter for transferring a controlled portion of the nucleator into the body of phase change material to induce crystallization and release of latent heat of crystallization. The special nucleator includes a component which, after the heating and cooling steps, will contain crystals for nucleating the phase change material and which has a solubility in the phase change material such that the controlled portion added to the body of phase change material will dissolve in that body during the heating step.

Implementation of Phase Change in Numerical Models of Heat Transfer

Abstract: This paper investigates some of the numerical problems involved in simulating heat transfer in porous media in the presence of phase change. Applications of this type of simulation include modeling of certain metal forming processes, of biological tissues and organs during cryosurgery or cyropreservation, and of heat transfer in frozen soils subjected to transient environmental conditions. A two-dimensional finite element model was used in which the latent heat is treated directly as an energy source in the problem formulation. Several parameters addressed in this work are crucial to the successful implementation of numerical methods for nonlinear heat transport with phase change, including: the effect of nodal point spacing on the occurrence and magnitude of numerical oscillations in the temperature solution and the use of grid point spacing to control these oscillations; the limiting element size which should be used in order to insure stable temperature fields; and the effect which the range of temperatures over which latent heat is liberated has on the solution. The results indicate that numerical stability is achieved for combinations of the foregoing parameters which yield small values of the Stefan number.

Vacuum evaporation apparatus

Abstract: A vacuum evaporation apparatus for depositing an evaporant as a thin film on a substrate comprises a sealed container including a substrate support for mounting thereon the substrate. A heat medium such as of diphenyl, for example, is filled in the substrate support. The heat medium in the substrate support is supplied with heat by a heater. When heated, the heat medium is vaporized and the vapor gas having absorbed heat of evaporation is moved quickly from a high-temperature region to a low-temperature region. The vapor gas in the low-temperature region is supersaturated and turned into the heat medium liquid. The heat energy born as latent heat by the vapor gas is given off to heat the substrate support uniformly. With this thermosiphon action, the substrate can be heated through the substrate support so as to have a uniform overall temperature distribution for forming a uniformly deposited thin film on the substrate surface.

Ethylene polymer composite heat storage material

Abstract: Heat storage composite consisting essentially of about 2-55% of an organic latent heat material and about 45-98% of a filled ethylene polymer.

Numerical Study of an AMTEX'75 Oceanic Cyclone

Abstract: The diabatic effects of latent heat release, boundary layer moisture and heat flux from the ocean surface and large-scale forcing due to upper-level systems are three physical processes affecting oceanic cyclogenesis. Detailed analyses of a major winter storm which occurred during the initial phase of the Air Mass Transformation Experiment in 1975 (AMTEX '75) indicated that the role of these three processes was vital to the Cyclone's development. To gain further insight into their influence, a control and three numerical experiments were performed using a multi-level moist primitive equation model with fine vertical resolution in the boundary layer. The simulation which included complete physics faithfully reproduced the major feature of the observed system. It was found that latent heating had a profound impact on the middle-level baroclinicity, the intensity and phase speed of the storm, and the vertical coupling within the simulated system. Without the surface moisture and heat source, the eff...

Asymmetries in evaporation and condensation Knudsen layer problems

Abstract: Basic asymmetries are shown to exist between half‐space problems for evaporation and those for condensation, in the regime of linear transport. The asymmetry is determined by the normalized latent heat of the substance, which is found to have a critical influence upon the flow behavior in the condensation problem. The analysis is based upon the linearized Maxwell–Boltzmann transport equation, and the results indicate nonexistence of solutions to the condensation half‐space problem for substances with latent heat below a certain critical value.

Influence of Moisture on Heat Transfer Through Fibrous-Insulating Materials

Abstract: In a previous study, the thermal conductivity of a high-density mineral fiberboard had been experimentally determined as a function of moisture content at ambient temperature in steady-state conditions, allowing moisture to be redistributed in the product. The results obtained were significantly different from those of other authors who had performed experiments with a uniform moisture distribution. The present work is an attempt to understand the reasons for these differences. A second series of tests has been performed on the same high-density product in an attempt to re-create the results of these authors. A method of measurement has been developed, and the results obtained for mean temperatures of 10 and 24/sup 0/C are presented and discussed. Special attention is given to the heat transfer caused by vapor diffusion and to the latent heat component of thermal conductivity due to this diffusion. A simple model for predicting the thermal conductivity in the conditions of the tests is presented; fairly good correlation with the experimental results was obtained. Finally, all results are compared and discussed in order to see which can be considered representative of field conditions.