Thermal conduction, unsteady liquids

THERMAL CONDUCTIVITY OF LIQUID REFRIGERANTS MEASURED BY AN UNSTEADY-STATE HOT-WIRE METHOD. 

Grassman P., Straumann W., Widmer F., Jobst W. Measurements of thermal conductivities of liquids by an unsteady state method.—In Progress in Intemat. res. on thermod. transp. prop. Princeton, 1962, p. 447—453. 

Unlike at adiabatic conditions, the height of the liquid level in a heated capillary tube depends not only on cr, r, pl and 6, but also on the viscosities and thermal conductivities of the two phases, the wall heat flux and the heat loss at the inlet. The latter affects the rate of liquid evaporation and hydraulic resistance of the capillary tube. The process becomes much more complicated when the flow undergoes small perturbations triggering unsteady flow of both phases. The rising velocity, pressure and temperature fluctuations are the cause for oscillations of the position of the meniscus, its shape and, accordingly, the fluctuations of the capillary pressure. Under constant wall temperature, the velocity and temperature fluctuations promote oscillations of the wall heat flux. 

Hence, the local mass transfer coefficient scales as the two-thirds power of a, mix for boundary layer theory adjacent to a solid-liquid interface, and the one-half power of A, mix for boundary layer theory adjacent to a gas-liquid interface, as well as unsteady state penetration theory without convective transport. By analogy, the local heat transfer coefficient follows the same scaling laws if one replaces a, mix in the previous equation by the thermal conductivity. 

MEASUREMENT OF THERMAL CONDUCTIVITIES OF ORGANIC ALIPHATIC LIQUIDS BY AN ABSOLUTE UNSTEADY-STATE METHOD. 

Typical values of thermal conductivities and thermal diffusivities in gases, liquids, and solids are given in Table 20.4-2. Some of the values are expected from experience for example, the thermal conductivities of metals are much higher than those of liquids or gases. Less obviously, the thermal diffusivities of nonmetallic solids and liquids are more nearly the same, indicating that unsteady heat transfer proceeds at more similar rates in these materials. 

In the emulsion phase/packet model, it is perceived that the resistance to heat transfer lies in a relatively thick emulsion layer adjacent to the heating surface. This approach employs an analogy between a fluidized bed and a liquid medium, which considers the emulsion phase/packets to be the continuous phase. Differences in the various emulsion phase models primarily depend on the way the packet is defined. The presence of the maxima in the h-U curve is attributed to the simultaneous effect of an increase in the frequency of packet replacement and an increase in the fraction of time for which the heat transfer surface is covered by bubbles/voids. This unsteady-state model reaches its limit when the particle thermal time constant is smaller than the particle contact time determined by the replacement rate for small particles. In this case, the heat transfer process can be approximated by a steady-state process. Mickley and Fairbanks (1955) treated the packet as a continuum phase and first recognized the significant role of particle heat transfer since the volumetric heat capacity of the particle is 1,000-fold that of the gas at atmospheric conditions. The transient heat conduction equations are solved for a packet of emulsion swept up to the wall by bubble-induced circulation. The model of Mickley and Fairbanks (1955) is introduced in the following discussion. 

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