Showing papers in "Journal of Heat Transfer-transactions of The Asme in 1978"

Two Thermodynamic Optima in the Design of Sensible Heat Units for Energy Storage

Abstract: The paper presents a treatment of sensible-heat energy storage units as systems intended to store useful work. An analysis of the thermodynamic irreversibilities associated with storing energy from a hot gas source as sensible heat in huge liquid baths points out two important trade-offs. First, there exists an optimum well-defined quantity of hot gas to be used in order to maximize the useful work stored in the liquid bath. Using more than this optimum quantity in the hope of maximizing the amount of thermal energy stored as sensible heat leads to severe thermodynamics losses. Second, there exists an optimum relationship among the gas-liquid heat-exchanger design parameters which minimizes the system irreversibility while maximizing its capability of storing useful work. This relationship provides a procedure for estimating the heat-exchanger optimum number of transfer units. The existence of the two optima demonstrates that designing sensible heat units for maximum thermal energy storage does not necessarily amount to thermodynamically optimizing such systems.

Numerical Method for Multi-Dimensional Freezing Problems in Arbitrary Domains

Abstract: This paper presents a simple numerical method for solving two and three-dimensional freezing problems with arbitrary geometries. The change of variable method introduced by Landau for the one-dimensional problem is extended to the multi-dimensional using an independent variable which takes constant values at the boundary and the freezing front. Example calculations were performed for the Stefan type freezing problem in regular squares, triangles, and ellipses. Then some of the results were compared with the experimental ones that were obtained for the constant cooling rate.

An Investigation of the Minimum Film Boiling Temperature on Horizontal Surfaces

Abstract: The frequency and magnitude of liquid-solid contacts in the film boiling of saturated ethanol and water on horizontal flat stainless steel and copper surfaces are examined with electrical conductance probes. It is observed that, at atmospheric pressure, contacts occur over a wide temperature range and are generally induced by hydrodynamic instabilities. In the ethanol system, these contacts can account for the entire heat transfer rate at the minimum film boiling temperature. The area and duration of contacts are strongly influenced by the dominant nucleation process, and thus depend on the interface temperature and wettability of the solid. At elevated pressures, direct liquid-solid contacts in film boiling are essentially nonexistent for stainless steel and the first major contact corresponds to quenching of the surface. Under these conditions, spontaneous nucleation upon contact seems to be the controlling mechanism for the minimum film boiling temperature. The minimum point appears to be determined either by a Taylor instability vapor removal limitation or by spontaneous nucleation. The governing mechanism is the one which is stable at the lowest wall temperature.

Natural Convection in a Horizontal Porous Medium Subjected to an End-to-End Temperature Difference

Abstract: Natural convection in a porous medium filling a slender horizontal space with an end-to-end temperature difference is studied analytically. The end-to-end temperature difference gives rise to a horizontal counterflow pattern augmenting the heat transfer rate through the porous medium. Two basic geometries are considered: horizontal layer confined between 2 adiabatic and impermeable parallel plates, and horizontal cylinder surrounded by an adiabatic and impermeable cylindrical surface. Nusselt number relations are derived in terms of the Rayleigh number and the cavity aspect ratio. The end-wall permeability is shown to affect the heat transfer rate through the medium. (12 refs.)

Thermosyphon models for downhole heat exchanger applications in shallow geothermal systems

Abstract: The analysis of downhole heat exchangers used to extract energy from relatively shallow geothermal wells leads to the consideration of several interesting problems of buoyancy-driven heat transfer in enclosures. This paper considers thermosyphoning through and around the wellbore casing which is perforated at two or more depths. Analytical models are developed for thermosyphoning in the cased well both with and without a heat exchanger installed. Theoretical results are compared with experimental values. These comparisons show that the observed energy extraction rates and flow rates through the well casing are possible with thermosyphoning as the only circulation mechanism within the well bore. The model with a heat exchanger installed is parametrically evaluated to illustrate the sensitivity of the model to estimated parameters and the effect of changes in design variables or constraints.

Bubble Growth in Variable Pressure Fields

Abstract: The case of a spherical vapor bubble growing in an infinite, uniformly heated liquid, has been analyzed under the thin boundary layer approximation for the effects of a variable pressure effects can be quite important and dominate the rate of growth. For the case where pressure changes cause the vapor temperature to behave as t/sup n/, (t being time), the bubble radius will grow as t/sup n+1/2/, significantly faster than the ..sqrt..t behavior usually expected. The analysis has been shown to compare favourably with existing data.

Simultaneous Measurements of Velocity and Temperature in Nonisothermal Flows

Abstract: A two-wire probe technique has been developed for the simultaneous and continuous measurements of instantaneous velocity and temperature in nonisothermal flows. The equations of the thermal equilibrium of two adjacent fine wires, one hot and one only a little above the fluid temperature, are solved by employing analog technique, wherein not only the difference of the heat transfer coefficients between two wires but also their variations with both the velocity and temperature are taken into consideration. The measuring system proposed in this study is founded on fairly strict principles of analysis and, hence, provides automatic compensation even for very large amplitude velocity and temperature fluctuations over a frequency range of d-c to six kHz.

Thermal Diffusivity of Dispersed Materials

Abstract: Measurements have been made of the effective thermal diffusivity at room temperature of composites consisting of one phase randomly dispersed in a second phase. The method is based on the flash technique. Data are presented for four types of composites ranging in particle-to-matrix diffusivity ratios from 0.48 to 1137; in volume specific heat ratios 0.04 to 1.16 and in volume fraction of dispersed particle from zero up to 34%. The results show that the limitations of the concept of an effective thermal diffusivity are far beyond the stiuations to which it is currently applied in the transient state heat conduction problems. Values of effective diffusivities derived from values of the effective thermal conductivity calculated from the Bruggeman variable-dispersion equation are found to agree well with the measured diffusivity values.