Processes in which heat is transferred to a cold surface with the simultaneous deposition of frost are important in a variety of refrigeration installations. As the frost layer grows, the heat transfer is affected in part because of the insulating effect of the frost. This can adversely affect the performance of cooling coils and plate freezers in domestic and industrial refrigeration installations. Previous studies indicate that the initial frost deposits are desirable since the rough frost surface acts as fins, thus temporarily increasing the heat transfer rate. The overall process is characterized by its time-dependent nature, especially during the early deposition periods. The work reported here attempts to examine this transient phenomenon by mathematically modeling the frost formation and heat transfer processes on plate freezers held at subfreezing temperatures. This will be achieved by combining three limiting solutions to the unsteady governing equations of the air-frost boundary layer in order to be able to draw an overall picture for the frost formation and heat transfer mechanisms with time.

Porous and granular materials often defy an accurate description which would identify them. Property values for such media reported in the literature can then serve as guides only and a user of such material who needs accurate property values has to obtain them by measurement on a sample of the specific material with which he is concerned. It is the purpose of the present paper to describe a device by which specific heat, heat conductivity, and thermal diffusivity of particulate materials can be measured rapidly and accurately. The sample has an annular cylindrical shape, and is heated with a uniform heat flux on its inside and outside surfaces. The analysis performed has resulted in closed form expressions for the properties in terms of the measured variables in the quasi-steady state regime. An apparatus was designed and manufactured to verify the analysis. The conductivity of dry sand obtained by this method agrees with that measured in a steady state test within 2.17%.

Les processus impliquant le transfert de chaleur d'un courant d'air humidḋe en éċoul≐ment lȧmiṅaiṙe à u̇n≐ plaqu̇e ḣorizontale accompagné d'un dépôt de givre sont importants dans divers appareils frigorifiques. La couche de givre qui s'accumule entrave le flux de chaleur vers la surface froide. Le présente étude a été effectuée pour déterminer théoriquement et expérimentalement les facteurs ayant une influence sur la formation de givre sur une surface froide et pour corréler les nombres de Nusselt et de Sherwood avec ces facteurs.

Experiments were performed to study the laminar developing and fully developed flow and heat transfer inside an elliptical duct having an aspect ratio of 0.5. The working fluid was air and two thermal situations were investigated, the first with the duct at a uniform temperature and the second when the wall temperature distribution is linear in the axial direction and does not vary transversely. The hydrodynamic results are presented in the form of a sequence of velocity profiles on the major and minor axes, measured at axial locations extending from the duct entrance to the fully developed regime. The axial drop in the static pressure due to the combined effect of the flow development and wall friction is also reported. The extended length necessary for static pressure development, expressed as xReDh, was found to be 0.0345. The thermal information depicts the temperature development in the duct entrance by a series of temperature profiles on the major and minor axes. The thermal results encompass as well the Nusselt number and the thermal entrance length in each of the above two thermal situations. To the author's knowledge, theoretical solutions for the hydrodynamic flow development in the entrance of elliptical ducts do not exist. The present experimental fully developed dimensionless velocity and friction factor were compared to the analytic value of L. N. Toa [On some laminar-forced-convection problems, ASME J. Heat Trans.83, 466–472 (1961)]. The percentage difference in the friction factor is 0.78%. The thermal development of the flow in the elliptical duct was studied analytically by N. T. Dunwoody [Thermal results for forced heat convection through elliptical ducts, J. appl. Meal. 29, 165–170 (1962)] and V. Javeri [Analysis of laminar thermal entrance region of elliptical and rectangular channels with the Kantorowich method, Wärme Stoffubert.9, 85–98 (1976)] for the uniform and linear wall temperature ducts respectively. Both analyses assume either uniform or fully developed velocity profiles in the developing regime.

Hydrodynamic and thermal characteristics of the fully developed laminar flow and heat transfer in an arbitrarily shaped triangular duct are evaluated using a finite difference technique. The hydrodynamic information encompasses the friction factor, the length from the tube entrance necessary for complete hydrodynamic development, and the incremental pressure drop due to flow development in the entrance section. The Nusselt numbers in the case of an allover isothermal ductNu T , as well as for a duct heated by an axial uniform heat flux while its transverse local periphery is at a constant temperatureNu H , are presented. Comparison of the isosceles results with those from the work of Shah [1], Sparrow and Haji-Sheikh [2], and Schmidt and Newell [3] revealed a maximum difference of about −0.2% in theNu Hi data, less than ±0.5% in theNu T ,about +0.3% in the friction factor, a −0.47% in the incremental pressure drop, and around −1% in the developing entrance length. The deviations from the results of other authors become smaller as the triangular geometry approaches the equilateral.

Removal of frost from an jupward facing horizontal cooling plate at a subfreezing temperature using a hot water spray is studied experimentally. The variation of the frost layer with time is approximately linear. A simple analytical model predicts the trend of the frost thickness-time change.

Macroencapsulation of phase change material (PCM) is one of the enhancement techniques to maximise heat transfer area between PCM and the surrounding heat transfer fluid (HTF). However, the selection of suitable encapsulation material, that can address the volumetric expansion problem and does not react with the core PCM, while being stable at elevated temperature, possesses a challenge. In this work, the thermo-mechanical characterisation of polymeric material is reported, which can be used for medium temperature (200–350 °C) solar applications. Three materials, such as polytetrafluoroethylene (PTFE), polyether ether ketone (PEEK) and polyether ketone ketone (PEKK) are chosen as an encapsulation material for storing a commercially available organic PCM, A164 in the capsule. Capsules of three materials are fabricated and the mechanical properties of the materials are evaluated after every 10 accelerated thermal cycles near the degradation temperature of the materials to understand their behaviour in extreme conditions. Dimensional and weight analyses of the macro encapsulated capsules is also performed to gauge any change in their physical properties. Thermo-mechanical properties show that the polymers chosen can be used as an encapsulation material for solar applications. Dimensional and weight analysis indicate no significant change in the properties of the polymers. High-temperature stability of these polymers even for extended duration indicates a maximum of 3 wt% change only. The compression test on these materials reveals that the PEKK can be used up to 275 °C with Young’s modulus decreases to 0.4 GPa. These results would form the ground for their applications as PCM-encapsulating materials targeting medium temperature solar applications.

For conditions encountered in much of the Midwest, an extensive layer of dry sandstones protected by an impermeable limestone roof is accessible for long-term (six-month), high-temperature (T > 100 °C) thermal energy storage. The performance of such storage beds is examined when the thermal properties of sandstones are modified so as to adjust the permeability towards an optimum in order to reduce pumping power. The thermal properties of consolidated St. Peter sandstone, dispersed St. Peter sand, and pelletized St. Peter sand are measured by several techniques. A nondimensional computer model for beds bounded by a constant temperature water table is employed to estimate the influence of thermal property manipulation of storage system performance. Disrupting the sandstone permits a substantial increase in the volume efficiency and first and second law efficiencies. Pelletizing sand into particles of approx. 1 mm diameter permits marginal further increases in performance. Disruption of the consolidated sandstone reduces the thermal diffusivity and increases the permeability. Both effects improve performance.

An analysis has been made to determine the successive stages of development as the natural convection boundary layer on a steadily heated vertical plate evolves into a plume. Both the wall plume and the free plume are investigated. The wall plume develops along an adiabatic wall which is the vertical extension of the heated plate. The free plume is created as the boundary layer streams away from the upper edge of the plate. Since the plate is heated on only one of its faces, the free plume is initially unsymmetric. The development of these plumes does not admit similarity-type boundary layer solutions, and numerical techniques were, therefore, employed, with results being obtained for Prandtl numbers of 0.7, 2, 5, and 10. It was found that at sufficient downstream distances both plumes attain their respective fully developed behaviors (i.e., similar profiles at successive streamwise stations). For the wall plume, the development for all Prandtl numbers is completed at a position that is about five plate lengths above the leading edge of the heated plate. The development length for the free plume for Pr = 0.7 is about the same as that for the wall plume, but about 30 plate lengths are required for the development of the free plume when Pr = 10. The fully developed free plume is symmetric.

In connection with energy conservation studies by subsurface construction, thermal properties of soils are needed for a wide range of temperature and moisture levels. Because of several disadvantages of steady state methods, including the problem of moisture migration, a transient method has been developed which requires substantially less time than steady state tests, reduces the problem of moisture migration, and provides simultaneously thermal conductivities and thermal diffusivities. The design of the apparatus and the test procedure are discussed. The agreement of sample results with steady state measurements assures reliability of the equipment which will be used for generating a wide spectrum of thermal soil data.Studien über unterirdische Bauwerke zur Energieersparnis erfordern die Kenntnis der thermischen Eigenschaften von Böden in einem weiten Temperatur- und Feuchtebereich. Wegen einiger Nachteile der stationären Methoden, darunter auch der Feuchtigkeitswanderung, wurde eine nichtstationäre Methode angewendet, die weniger Zeit braucht, keine Verschiebung der Feuchtigkeit zeigt und zugleich Wärmeleitfähigkeit und Temperaturleitfähigkeit liefert. Konstruktion der Apparatur und Meßverfahren werden mitgeteilt. Die gute Übereinstimmung mit den Ergebnissen stationärer Messungen zeigt die Brauchbarkeit der Anordnung, mit der ein breiter Bereich thermischer Daten von Böden ermittelt werden soll.

An instrument which measures the product of density, specific heat, and thermal conductivity of solid materials is described, a parameter which frequently occurs in analyses of unsteady heat conduction processes. The results of measurements are reported which demonstrate the accuracy which can be obtained.

In connection with energy conservation studies by subsurface construction, thermal properties of soils are needed for a wide range of temperature and moisture levels. Because of several disadvantages of steady state methods, including the problem of moisture migration, a transient method has been developed which requires substantially less time than steady state tests, reduces the problem of moisture migration, and provides simultaneously thermal conductivities and thermal diffusivities. The design of the apparatus and the test procedure are discussed. The agreement of sample results with steady state measurements assures reliability of the equipment which will be used for generating a wide spectrum of thermal soil data.

The laminar flow and heat transfer on a flat plate subjected to nonuniform velocity and temperature profiles in the approaching free stream have been analyzed. The plate is situated in the laminar wake of an upstream plate. The extent of the approach-flow nonuniformities depends on the streamwise length of the wake relative to the length of the upstream plate. It was found that the effect of the nonuniformities is to reduce the wall shear and heat transfer on the downstream plate relative to their values for a uniform approach flow. Reductions of up to fifty percent were encountered. The extent of the reductions diminishes with increasing downstream distance, but non-negligible effects persist to a considerable length along the plate.

The effect of an unheated starting length on combined forced and natural convection adjacent to a vertical plate has been investigated by solving the nonsimilar laminar boundary layer equations. The solutions were carried out numerically for prescribed values of the governing parameters which include the starting length Reynolds number Re0, a mixed convection parameter gβ(ΔT)ν/U∞3, and the Prandtl number (which was assigned a value of 0.7). The local heat transfer results show that the presence of the unheated starting length can significantly accentuate the effects of buoyancy relative to the case of no starting length. The degree of accentuation of the buoyancy effects is strongly influenced by the magnitude of gβ(ΔT)ν/U∞3. When this parameter is on the order of 10−3, the natural convection contribution to the heat transfer coefficient is markedly increased owing to the starting length. On the other hand, when gβ(ΔT)ν/U∞3 is about 10−5 the buoyancy contribution is essentially unaffected by the starting length. The shape of the velocity profile is also found to be highly responsive to the interaction between the buoyancy and the starting length. As a by-product of the research, the accuracy of a well-known integral momentum/energy solution for pure forced convection with a starting length was established. In addition, velocity profiles for mixed convection without a starting length were compared with those of experiment in order to appraise a proposed explanation for a disparity that had been previously identified in the literature.