Subcooling

About: Subcooling is a research topic. Over the lifetime, 6150 publications have been published within this topic receiving 99125 citations.

A new type of diabatic flow pattern map for boiling heat transfer in microchannels

Abstract: Flow pattern data and bubble measurements for two small diameter sight glass tubes are used to propose a new type of flow pattern map for evaporating flows in microchannels. Rather than segregating the observations into the traditional flow regimes and an adiabatic map, the new diabatic map classifies flows into three types: (i) the isolated bubble regime, where the bubble generation rate is much larger than the bubble coalescence rate and includes both bubbly and slug flows, (ii) the coalescing bubble regime, where the bubble coalescence rate is much larger than the bubble generation rate and exists up to the end of the coalescence process and (iii) the annular regime, whose extent is limited by the vapor quality at the onset of critical heat flux. This formulation is thought to be more useful for phenomenological modeling of the processes controlling boiling heat transfer and two-phase pressure drops in microchannels, and it also visually defines the feasible operating limit of microchannel heat spreaders at the critical vapor quality corresponding to critical heat flux (CHF). The database covers two refrigerants (R-134a and R-245fa) and two channel diameters (0.509 and 0.790 mm). The micro-evaporator length was varied from 20 to 70 mm, the inlet subcooling from 2 to 15 °C, the mass flux from 200 to 2000 kg m−2 s−1 and heat fluxes up to 597 kW m−2. Three different saturation temperatures were tested: 26, 30 and 35 °C.

Determination of the enthalpy of PCM as a function of temperature using a heat-flux DSC—A study of different measurement procedures and their accuracy

Abstract: Thermal energy storage by latent heat allows storing high amounts of energy working in narrow margins of temperature. The use of phase change material (PCM) for the latent heat storage has been studied in different applications and it has been commercialized in containers to transport blood, products sensible to temperature, to decrease their energy demand. The use of PCM in cooling and refrigeration has been attracting a lot of interest lately, but for all applications, the properties of these materials need to be known with sufficient accuracy. Regarding heat storage, it is necessary to know the enthalpy as a function of temperature. The most widely used calorimeter is the heatflux differential scanning calorimetry (hf-DSC). The objective of this study is to investigate different methods for hf- DSC analysis, namely the dynamic method and the step method, and to test their accuracy in the determination of enthalpy–temperature relationship of PCM. For the dynamic method, a strong influence of heating/cooling rate was observed. For the step method, the resulting enthalpy–temperature relationship is independent of heating/cooling rate. Commercial PCM RT27 was chosen as sample material to avoid subcooling and kinetic effects in the test measurements. The approach introduced in this study can be used to carry out similar investigations for other classes of PCM and/or other DSC instruments.

The onset of flow instability in uniformly heated horizontal microchannels

Abstract: Onset of nucleate boiling and onset of flow instability in uniformly heated microchannels with subcooled water flow were experimentally investigated using 22-cm long tubular test sections, 1.17 mm and 1.45 mm in diameter, with a 16-cm long heated length. Important experimental parameter ranges were: 3.44 to 10.34 bar channel exit pressure; 800 to 4,500 kg/m{sup 2}s mass flux (1 to 5 m/s inlet velocity); 0 to 4.0 MW/m{sup 2} channel wall heat flux; and 7,440--33,000 Peclet number at the onset of flow instability. Demand curves (pressure drop versus mass flow rate curves for fixed wall heat flux and channel exit pressure) were generated for the test sections, and were utilized for the specification of the onset of nucleate boiling and the onset of flow instability points. The obtained onset of nucleate boiling and onset of flow instability data are presented and compared with relevant widely used correlations.

The effect of void fraction correlation and heat flux assumption on refrigerant charge inventory predictions

Abstract: Ten void fraction correlations and four heat flux assumptions are evaluated for their effect on refrigerant charge inventory predictions Comparisons between mass inventory predictions are made for condensers and evaporators over representative heat pump operating ranges of saturation temperature, mass quality, and mass flux The choice of void fraction model is found to have a major effect on refrigerant inventory prediction The maximum variation of predictions ranges from a factor of 10 for low-ambient, heating-mode evaporators to 42 for cooling-mode evaporators and 17 for high-ambient cooling-mode condensers assuming no subcooling The correlations of Hughmark, Premoli, Tandon, and Baroczy are found to give the highest predictions and closest agreement to measured total system charge The choice of heat flux assumption is shown to be insignificant for forced-flow evaporators and of secondary to possibly equal importance to choice of void fraction model for condensers Implications for charge balancing, off-design and transient performance prediction, and unit reliability are discussed