Heat flow phase changes

A buffer of compacted bentonite is planned to be used to prevent the movement of groundwater and the consequential escape of material from a geological repository for spent nuclear fuel. Fluid flow, phase changes, mechanical behaviour of the buffer, rock, and the waste canisters, and the heat produced by the waste constitute a coupled thermohydromechanical system. The aim of the study is to derive a general thermodynamically consistent THM model for an arbitrary mixture. The general theory is applied to the thermohydraulic modelling of a mixture of compacted bentonite, liquid water, vapour, and air. 

Differential scanning calorimetry DSC Power difference or heat flow Heat capacity, phase changes, reactions... 

Figure 2.17. Quasi-isothermal cure at 80°C of an epoxy(/ = 2)-amine(/ = 2) (unmodified u) and epoxy(/ = 2)-amine(/ = 2)/20% PES (modified m) (a) non-reversing heat flow (b) change in heat capacity and heat flow phase.

Figure 2.112. Nonreversing (NR) heat flow (a), heat capacity change (ACp) (b), and heat flow phase (( )) (c) for the isothermal cure of stoichiometric diglycidyl ether of bisphenol A (DGEBA) and methylenedianilme (MDA) mixture at 70/80/100 °C the increase in ACp due to reaction and the stepwise decrease due to vitrification are indicated on the graph (1 °C/60s) [data reproduced fromSwier et al. (2004) with permission of John Wiley Sons, Inc.].

Figure 2.116. Nonreversing heat flow, change in heat capacity, and heat flow phase signal from MTDSC and percent of hght transmittance from optical microscopy (OM) for the reactive blend DGEBA + aniline (r = l)/20 wt% PES cnred at 100/90/80°C the cloud point from OM (A), onset of heat flow phase relaxation corresponding to vitrification of PES-rich phase (O) and epoxy-amine-rich phase ( ) are shown and also indicated in the heat flow and heat capacity signals [reprinted from Swier and Van Mele (2003a) with permission of Elsevier Ltd.].

Differential Scanning Calorimetry DSC Heat flow difference Heat capacity Phase changes (melting etc) Glass transition... 

Differentiai scanning caiorimetry (DSC) Heat flow Heat capacity Phase changes Reactions... 

Fig. 17. Heat-transfer coefficient comparisons for the same volumetric flow rates for (A) water, 6.29 kW, and a phase-change-material slurry (O), 10% mixture, 12.30 kW and ( ), 10% mixture, 6.21 kW. The Reynolds number was 13,225 to 17,493 for the case of water.

The plate dryer is limited in its scope of apphcations only in the consistency of the feed material (the products must be friable, free flowing, and not undergo phase changes) and diying temperatures up to 320°C. Applications include speci ty chemicals, pharmaceuticals, foods, polymers, pigments, etc. Initial moisture or volatile level can be as high as 65 percent and the unit is often used as a final dryer to take materials to a bone-dry state, if necessary. The plate dryer can also be used for heat treatment, removal of waters of hydration (bound moisture), solvent removal, and as a product cooler. 

These refer to hot and cold fluid terminal temperatures, inlet of one fluid versus outlet of the other. For a cross exchanger with no phase change, the ATm gives exact results for true countercurrent flow. Most heat exehang-ers, how ever, deviate from true countercurrent so a correction factor, F, is needed. 

Tlic heat duty is best calculated with a process simulation program hi will account for phase changes as the fluid passes throiigli ilic ctioke. It will balance the enthalpies and accurately predict the change m tcnipcrature across the choke. Heat duty should be checked for vanoits combinations of inlet temperature, pressure, flow rate, and outlet temper ature and pressure, so as to determine the most critical combination. 

Thermochemistry is concerned with the study of thermal effects associated with phase changes, formation of chemical compouncls or solutions, and chemical reactions in general. The amount of heat (Q) liberated (or absorbed) is usually measured either in a batch-type bomb calorimeter at fixed volume or in a steady-flow calorimeter at constant pressure. Under these operating conditions, Q= Q, = AU (net change in the internal energy of the system) for the bomb calorimeter, while Q Qp = AH (net change in the enthalpy of the system) for the flow calorimeter. For a pure substance. 

Most of the remainder of this chapter is devoted to a discussion of the magnitude of the heat flow in chemical reactions or phase changes. However, we will focus on a simpler process in which the only effect of the heat flow is to change the temperature of a system. In general, the relationship between the magnitude of the heat flow, q, and the temperature change, At, is given by the equation... 

Phase changes are examples of (c). In order to melt a mole of ice in contact with air at 298.15 K, heat must flow into the system from the air. The increase in entropy of the system is 22.00 J K-1 - mol-1. The heat leaving the air decreases its entropy by 20.15 J K-1 mol-1. The net change in the universe is once again positive ... 

The problems of micro-hydrodynamics were considered in different contexts (1) drag in micro-channels with a hydraulic diameter from 10 m to 10 m at laminar, transient and turbulent single-phase flows, (2) heat transfer in liquid and gas flows in small channels, and (3) two-phase flow in adiabatic and heated microchannels. The smdies performed in these directions encompass a vast class of problems related to flow of incompressible and compressible fluids in regular and irregular micro-channels under adiabatic conditions, heat transfer, as well as phase change. 

Understanding the differences in two-phase flow characteristics between conventional size channels and micro-channels is also important for designing mini- or micro-heat exchangers, since the flow characteristics will affect the phase change heat transfer. 

Carey van P (1992) Liquid-vapor phase-change phenomena. An introduction to the thermophysics of vaporization and condensation processes in heat transfer equipment. Hemisphere, New York Celata GP, Cumo M, Mariani A (1997) Experimental evaluation of the onset of subcooled flow boiling at high liquid velocity and subcoohng. Int J Heat Mass Transfer 40 2979-2885 Celata GP, Cumo M, Mariani A (1993) Burnout in highly subcooled water flow boiling in small diameter tubes. Int J Heat Mass Transfer 36 1269-1285 Chen JC (1966) Correlation for boiling heat transfer to saturated fluids in convective flow. Ind Eng Chem Process Des Develop 5 322-329... 

Carey VP (1992) Liquid-vapour phase-change phenomena. Hemisphere, Washington, DC Collier SP (1981) Convective boiling and condensation. McGraw-Hill, New York Ha JM, Peterson GP (1998) Capillary performance of evaporation flow in micro grooves an analytical approach for very small tilt angles. ASME J Heat Transfer 120 452 57 Hetsroni G, Yarin LP, Pogrebnyak E (2004) Onset of flow instability in a heated capillary tube. Int J Multiphase Flow 30 1424-1449... 

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