Analogy in the processes of heat exchange of capillary-porous coatings in energy installations

Abstract

A model of the dynamics of steam bubbles generating on a solid surface in porous structures and a steam-generating wall (substrate) is developed. The model is based on the filming and photography with speed camera SKS-1M. The removal of high heat fluxes (up to 2х106 W/m2) is provided by the combined action of capillary and mass forces with application of intensifiers. An analytical model is developed based on the theory of thermoelasticity. The limiting state of a poorly heat-conducting porous coating and a metal substrate has been determined. The heat fluxes were calculated from the time of spontaneous appearance of the steam nucleation (10-8) up to the time of material destruction (102 ÷ 103 s). The destruction of the coating under the action of compression forces occurs in much earlier time than the tension forces. The intervals of the heat flux within which such destruction occurs are different for the quartz coating qmax ≈ 7х107 W/m2, qmin ≈ 8х104 W/m2 and for granite coating qmax ≈ 1х107 W/m2, qmin ≈ 21х104 W/m2. Experimental units, experimental conditions, the results of the heat exchange crisis and the limiting state of the surface are presented, and critical heat fluxes are calculated. The investigated capillary-porous system, operating under the combined action of capillary and mass forces, has the advantage over pool boiling, thin-film evaporators and heat pipes.

Heat transfer for the boiling crisis in porous systems

“…In our research we used a rocket-type flame-jet burner (marked with 1 on figure 1) with a porous cooling system 2 and with 3 is denoted the casing. With such rocket -type jet burner can be produced different heat transfer intensifiers for cooling of nozzles and combustion chambers [13,15]. Burner (figure 2) Combustion chamber 1, a Laval nozzle 2 with confusor 3 and diffuser 4 parts and critical section 5 between them, swirl 6, atomizer 7 and distribution head 11 with branch pipes 12, 13, 14 for supply of oxidizer 19 and fuel 20 to combustion chamber 1 and coolant 18 to the cooling cavity 10 formed between shell 8 and the outer walls of combustion chamber 1 and Laval nozzle 2, installed in the housing 8.…”

“…In the production of porous materials one of the main characteristics of processing and application is stated by the structure of the pores of the material. The porous materials are robust and resistant to corrosion, with wide operating temperature range, don't clog the filtered liquid, machinability, allow multiple regeneration, have high electrical and thermal conductivity [11][12][13][14][15].…”

Study heat exchange in porous structures

“…Under certain conditions, followed by the forming of a "dry spot" [20]. The value of the detachment radius R 0 is calculated by the equation provided in [25]. After some "silence" of the generation, a new bubble spontaneously emerges, with growth time controlled by the coolant flow rate decreasing and occurs with a more intense heat supply from the thin superheated liquid layer.…”

“…In [19][20][21][22][23][24][25] are summarized the experimental data on different thermohydraulic processes. Calculated of heat load (equation 1-3) on temperature pressure, liquid excess, pressure, thermophysical properties of the wall, orientation, geometric characteristics of porous structures and heat exchange wall are obtained.…”

“…It has been found the most effective and reliable way to intensify heat exchange during boiling is the use of capillary-porous coatings. In this way, the operating mode and technological indicators are improved at a low price [19][20][21][22][23][24][25].…”