Mieczysław Poniewski

Hysteresis Phenomena at the Onset of Subcooled Nucleate Flow Boiling in Microchannels

In this paper, attempts were made to experimentally investigate the boiling incipience in a narrow rectangular vertical channel of 1 mm depth with an external 40 mm wide wall heated uniformly and others assumed quasiadiabatic. The “boiling front” location was determined from the temperature distribution of the heated wall obtained from liquid crystal thermography. Boiling incipience occurs when a considerable rise in the wall temperature above the saturation temperature takes place. Thus, boiling incipience is accompanied by “nucleation hysteresis.” The impact of various factors on the boiling incipience in microchannels, such as pressure, the inlet liquid subcooling, and flow velocity, were investigated.

Experimental Error Analysis and Heat Polynomial Method Improvement for Boiling Heat Transfer Numerical Calculations in Minichannels

The paper continues the discussion of experimental and numerical investigations of forced convection boiling heat transfer in vertical minichannels covered by two former editions of this conference and our previous papers. Liquid crystal thermography technique was used for measuring the two-dimensional heating surface temperature distribution and boiling front detection. Influence of selected parameters on boiling heat transfer and nucleation hysteresis was observed and discussed. The two-dimensional heat transfer model and the analytic-numerical heat polynomial method were applied to solve the inverse boundary value problem and determine the temperature distributions in the heating foil and protecting glass and the boiling heat transfer coefficient as well. This paper shows how to modify and improve the heat polynomial method if we know the measurement errors and implement them into the numerical procedure. The accuracy of temperature measurements on the heating surface with liquid crystal method was estimated and the analysis of experimental results was given. The functions sought in numerical calculations describe temperature distribution in the protecting glass and the heating foil of the minichannel. They are presented in the form of linear combination of heat polynomials. The adopted boundary conditions and temperature measurements are used to construct error functionals. The latter express the root-mean-square errors, with which computed solutions satisfy relevant boundary conditions. On the basis of functional minimalisation unknown coefficients of linear combinations are determined. The solutions obtained satisfy the differential equations in the exact manner whereas the adopted boundary conditions are met in the approximate fashion. The unknown boiling heat transfer coefficient is the function computed from the boundary condition of the third kind. In the modified method, measurement errors are weights for individual temperature measurements. The more accurate is the measurement, i.e. has a smaller error, the greater is the weight put to it in further calculations. Therefore, it is possible to heighten the accuracy with which glass and foil temperature distributions, determined experimentally, fulfil the assumed equality conditions on the contact surface. Temperature distributions in the glass and the foil, computed on the basis of the modified method, are closer to real values than those obtained with the basic one. Local heat transfer coefficients obtained for two-dimensional boiling heat transfer model with both the basic and the modified heat polynomial methods are also compared.Copyright

Influence of Selected Parameters on Boiling Heat Transfer in Minichannels

The experimental investigations cover heat transfer of refrigerants R 123 and R 11 flowing through vertical minichannels of 40 mm wide rectangular section and depths of 1 mm, 1.5 mm and 2 mm. The heating foil, supplied with controlled direct current, constitutes one of the surfaces of the minichannel. The liquid crystal thermography technique is applied in order to measure the two-dimensional temperature field of the heating surface. The investigations focus on the transition from single-phase forced convection to nucleate boiling, i.e. in the zone of boiling incipience. The present work aims to examine and analyze how the selected parameters (inlet pressure, inlet liquid subcooling, liquid flow velocity) affect nucleate boiling incipience for various geometry (changeable depth) of the minichannel. Furthermore, the investigations are intended to develop a correlation for the calculations of the Nusselt number under the conditions of boiling incipience in the minichannel. The equations are derived as modifications of the already developed ones [Piasecka, 2002; Piasecka and Poniewski, 2003b,c; Piasecka et al., 2004] and as a function of changeable parameters in the experimental investigations.Copyright

Photographic Technique Application to the Determination of Void Fraction in Two-Phase Flow Boiling in Mini Channels

The study deals with experimental investigations into heat transfer flow boiling in rectangular, vertical and horizontal mini-channels. The dedicated measurement system was designed to observe flow boiling in the mini-channel. The main task of the system is to record images of two-phase flow with a high-speed video camera, which is equipped with the synchronous movement system. The data, in the form of the footage of two-phase flow structures, were subjected to the analysis with the scripts, developed for that purpose in the MathWorks Matlab 2010b environment. The scripts allowed the determination of void fraction for each pattern of two-phase flow structures observed, at various heat fluxes and various volume fluxes, for two spatial orientations of the mini-channel: vertical and horizontal.

Statistic analysis of the boiling curve for a droplet

Abstract Statistic analysis of the experimentally determined values of a water droplets evaporation times was made. Measurements were taken from heating surface temperatures characteristic for liquid phase natural convection up to film boiling of a droplet. The results obtained confirm the hypothesis of two boiling curves in the region of transition boiling proposed by Witte and Lienhard. They also allow this concept on the region of nucleate boiling to be expanded.

Dissipative model of film boiling crisis

Abstract A dissipative model of the film boiling crisis based on the variational hypothesis of nonequilibrium phase change is presented. Transfer systems—characteristic for film and transition boiling of a liquid droplet on a plane horizontal and isothermal heating surface—were constructed. The value of the minimum film boiling temperature T p , min was calculated from the criterion of equality of local potentials for two competitive transfer systems. The curves p = p ( T p , min ) for hydrodynamic and thermodynamic models of the film boiling crisis for water have been determined and compared with the results achieved for the dissipative model.

Experimental Data Acquisition System for Flow Boiling in Mini Channels

The study discusses experiments on ethanol flow boiling in mini-channels. Pre-set thermal and flow parameters (pressure and temperature at the inlet, mass flux, the heat flux delivered to the heater) and temperature, pressure and void fraction measurements obtained in the experiment provided data for further simulation computations. On the basis of high-speed camera images, void fraction in the mini-channel was determined. Thermograms of the heater surface and the surface in the adiabatic part of the mini-channel allowed producing temperature profiles along the channel length. The read and computed parameters satisfy the expected dependences (especially that for void fraction as a function of volume flux and heat flux). Those were used to solve inverse heat conduction problems.