No abstract

It was shown that liquid boiling processes occur in the porous cooling systems of the elements of heat and power installations and crisis situation of the heat exchange wall overheating may occur at high thermal conditions. An experimental installation was assembled whose schematic and conditions of experimentation were represented in the paper studying the crisis. A crisis mechanism was developed. The gravitational potential aids in the destruction of steam conglomerates in a porous structure, facilitating the transport of an underheated liquid. A surplus of liquid in the porous system creates a directional movement of the flow, which leads to deformation of steam bubbles in the structure, a decrease in diameter, and an increase in the frequency of the formation of bubbles. As the flow velocity increases, the energy consumed for liquid displacement from the wall boundary-layer increases, and consequently, the rate of steam generation and the value of the critical flow increases. An increase of critical load will be achieved at a high flow velocity of the liquid, which will lead to an increase of consumption of energy that is used to power the pressure units. Equations are proposed for computing the hydrodynamic crisis, taking into account the combined actions of gravitational and capillary forces, creating surplus of liquid, underheating and additional velocity to the flow. Theoretical models are confirmed by experiments for a wide range of pressure changes in the system, the parameters of the capillary-porous structure and its orientation in a gravitational field.

Model of the vapour bubbles formed at the solid surface in porous structures and steam generating wall (base) was developed. The model is based on filming and photography by high-speed camcorder SKS-1M. Heat flow extraction (up to 2.106 Wm−2) is ensured by the joint action of capillary and mass forces with application of intensifier. The analytical model is based on the theory of thermoelectricity. Limit state of the poor conductive porous coating and metal base are determined. Heat flows were calculated from the time of spontaneous birth of vapour bubble (10-8) to the material destruction time (102÷103s), i.e. the time interval was described from the relaxation process to macro process (destruction). The size of pulled particles at the moment of porous coating destruction in model gave a good match with trial at the optic stand.

Holography and high-speed filming were used to reveal movements and deformations of the capillary and porous material, allowing to calculate thermo-hydraulic characteristics of boiling liquid in the porous structures. These porous structures work at the joint action of capillary and mass forces, which are generalised in the form of dependences used in the calculation for oil coolers in thermal power plants (TPP). Furthermore, the mechanism of the boiling process in porous structures in the field of mass forces is explained. The development process of water steam formation in the mesh porous structures working at joint action of gravitational and capillary forces is investigated. Certain regularities pertained to the internal characteristics of boiling in cells of porous structure are revealed, by means of a holographic interferometry and high-speed filming. Formulas for calculation of specific thermal streams through thermo-hydraulic characteristics of water steam formation in mesh structures are obtained, in relation to heat engineering of thermal power plants. This is the first calculation of heat flow through the thermal-hydraulic characteristics of the boiling process in a reticulated porous structure obtained by a photo film and holographic observations.

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.