WATER TWO-PHASE FLOW THROUGH A CONVERGENT-DIVERGENT NOZZLE WITH VARIABLE BACKPRESSURE: CHECK OF CALCULATION METHOD TO ESTIMATE MASS FLOWRATE AND CRITICAL FLOW CONDITIONS

Abstract - A careful design of the pressure safety valves (PSV) is an essential requisite for safeguarding of industrial plants; if two-phase flow is possible, it is very important for their correct design to predict the onset of the critical flow. This phenomenon occurs when the fluid velocity becomes equal or higher than the sound velocity in the fluid. Some other interesting aspects for the PSV design are the calculation of the subcritical two-phase flowrate and the back-pressure effect on the valve discharge performance. This paper shows the results of an experimental research carried out with steam-water two-phase through a convergent-divergent nozzle. The effect of some test parameters, such as inlet pressure and quality, mass flowrate, back-pressure, on the flow behaviour are evaluated. The experimental data are also compared with the results of a calculation method based on the homogeneous equilibrium model. The results show quite a good reliability of the model, with some uncertainties related to the test procedures (the mass flow rate is imposed); indeed, this condition makes difficult the checking of the critical flow arising, in spite of the help of the experimental data on the test section pressure profile

Ebollizione in convezione forzata in condizioni di microgravità

Rendiconti Accademia Nazionale delle Scienze detta dei XL Memorie di Scienze Fisiche e Natural

The effect of plate thickness, surface tension and fluid flow on detachment of drops from a plate

The pinch-off of drops from the downstream end of plates is an important re-entrainment mechanism of condensate in compact condensers. The present experimental study complements a previous one [1], in which ethanol and water drops were created for various mass densities and gas velocities at the downstream end of a plate that was fully wetted. In the present study, the effects of plate thickness and surface tension are studied with ethane drops detaching in ethane vapor flow at nearcritical conditions. If the thickness of the plate, D, is introduced as a governing length scale, the Bond number Dr g D2 / s is found to indicate whether or not the plate thickness affects drop detachment. A theoretical prediction for the drop size after pinch-off in otherwise quiescent vapor is compared with experiments, which results in a correlation valid for high Bond numbers that are based on the drop diameter. The predictive method presented in the previous study took account of the effect of flow on detaching drop size. It has now been extended and generalized. The combined effect of plate thickness and high vapor velocity is found to lead to a bifurcation in the main drop size after detachment. This phenomenon is due to the facts that the position of the drop foot underneath the plate is unstable, and that detachment near the rim of a plate is essentially different from that from the center of the plate

Experimental Analysis of the Microconvective Heat Transfer in the Laminar and Transition Regions

This article details experiments to determine the Nusselt number for laminar and tran- sitional liquid flows (water and FC-72) through rough stainless-steel microtubes of 440-, 15 280-, and 146-m inner diameter. Under laminar conditions, the average Nusselt number approaches the fully developed value for uniformly heated tubes as Reynolds decreases. For higher Reynolds, the region of thermal development increases the average convective heat transfer coefficient, which becomes a function of the Reynolds and Prandlt numbers and of the inner diameter-to-heated-length ratio. The effect of roughness is negligible in 20 the laminar regime. Under transition, the average Nusselt steeply increases with Reynolds, more than for conventional pipes

Experimental Analysis of the Convective Heat Transfer in the Transition Region of Microtubes

This paper reports the main findings of an experimental campaign aimed to determine the values of the Nusselt number for liquid flows through microtubes having a relative roughness up to 4% in the laminar and transitional regime. Three microtubes having an inner diameter of 440, 280 and 146 m uniformly heated by Joule effect with a constant power DC supply have been tested. The experimental results have evidenced that under laminar conditions the average Nusselt number approaches the fully developed constant value for uniformly heated tubes when the Reynolds number decreases. It is proven that the axial conduction along the fluid and the conjugate heat transfer to the walls can be neglected at low Reynolds numbers for the operative conditions adopted during the runs. For higher Reynolds numbers the region of thermal development exerts a stronger influence, increasing the average convective heat transfer coefficient, which becomes a function of the Reynolds number,of the Prandlt number and of the d/L (inner diameter-to-heated length) ratio. The experimental results of this work also demonstrate that in microtubes with a relative roughness lower than 4% the effect of the roughness on the Nusselt number can be neglected in the laminar regime. When the laminar-to-turbulent transition is reached the average value of the Nusselt number steeply increases with the Reynolds number. This behaviour is more pronounced for microtubes than for pipes of conventional size, where the onset of mixed convection smoothes the step variation of the average Nusselt number at the change in the flow regime

Experimental study on bubble detachment under variable heat load and the action of electric field

This paper is aimed at studying single bubble growth and detachment in controlled conditions and under the action of an electric field. An experimental set-up was built, consisting mainly of a stainless steel lamina with a nucleation site etched in its center, facing the testing fluid. The required heating power was provided by an SMD microresistor, stuck by means of epoxy resin on the back side of the plate. A signal generator, connected to the resistor, allowed for heating power modulation and thus for variation of the bubble size. Two different fluids, FC-72 and HFE-7100 were tested. An external electrostatic field was applied by imposing up to 15 kV dc to a ring of 4mm inner diameter suspended 3mm above the plate: in this way, a very clear axisymmetric configuration was created, suitable for two-dimensional analyses. High speed images were taken by means of a high-speed camera, operated at 1000 frames per second with a resolution of about 100 pixel/mm. Images were treated with a dedicated image processing software to obtain bubble significant geometrical parameters. The preliminary results reported here show elongation of the detached bubble under the action of the electric field, confirming former results obtained with nitrogen bubbles originating from an orifice in the same liquids. The detachment volume under the action of the electric field either increases or decreases, for low and high electric permittivity fluid, respectively, evidencing the complex nature and role of the electric forces. First results under variable heat load evidence a reduction in detachment diameter. The dynamical forces outside the microlayer region seem to have a minor role, compared to the static ones, in vapor bubble growth

The effect of the angle of inclination of a condenser on the gas-to-plate heat resistence in dropwise condensation

In a previous study [1] it was shown that tilting a compact plate condenser may increase the heat flux to the coolant by about 8%, depending on the inlet humidity. The explanation for this is enhanced drainage: the length of the paths of large drainage drops is increased, and the re-initiation of condensation at the ‘dry’ tracks decreases the mean gas-to-plate heat resistance. In the present study this explanation is verified, using infrared thermography and the following strategy of analysis [2]. The spatial temperature variations that occur in dropwise condensation on a condenser plate are measured instantaneously. They are averaged in time only in the direction parallel to the coolant flow (not vertical in a tilted exchanger). An assessment of extreme values leads to determination of the gas-to-plate heat resistance. The parameter Fcond quantifies the deviation of actual heat transfer from purely conductive heat transfer through the condensate. Mixing in the condensate, and the continual refreshing of liquid in contact with the gas, causes Fcond to be about 4 in a vertical condenser with dropwise condensation [2]. In the inclined condenser Fcond is found to be about 5, showing that tilting the exchanger affects the mean gas-to-plate heat resistance in the expected way