| The phenomenon of condensation with noncondensable gas widely exists in many industrial processes, such as nuclear industry, air conditioning and refrigeration industry, chemical process industry and seawater desalination industry. So far, most of the researchers have investigated this phenomenon only in vertical orientation. However, on some specific occasions, only horizontal in-tube condensation can be used as an example of titanium condenser in PTA production. Due to the condensate stratification and multidimensional nature, the mechanism of condensation in presence of noncondensable gas in horizontal tubes has not yet well understood.In this paper, the influence factors on condensation were analyzed. The distributions of local heat transfer characteristics were obtained. Then the heat transfer enhancement inside corrugated low finned tubes (CLFT) was studied. The effects of gas mixture inlet conditions and structural parameters were investigated. Finally the heat transfer model with non-uniform wall temperatures was developped. The reason why the heat transfer capacity gradually reduced along the tube was found. And the enhancement mechanism of condensation with noncondensable inside CLFTs were revealed. The main content and results are as follows:(1) The condensation in presence of noncondensable gas in horizontal tubes was studied by the method of temperature measurement within a large range of Reynolds number. The effects of noncondensable gas mass fraction, gas mixture mass flux, inlet pressure and coolant flow rate were analyzed. The results showed that heat transfer coefficient reduced by 59.32% when noncondensable gas fraction reached 30%. But during the condition of high Reynolds number, the gas-liquid interface shear stress could weaken the negative impact of the noncondensable gas. And for stratified flow, the local heat transfer coefficient at the top part was higher than that at the bottom owing to the asymmetrical liquid film. However, this gap gradually decreased along the tube, especially at higher inlet noncondensable gas fraction.(2) Based on the dimensionless analysis of heat transfer coefficient, two correlations respectively for stratified flow and annular flow regimes were established by fitting experimental data. And the modified Froude number was used as the indicator of transition from stratified to annular flow. The predicted heat transfer coefficients were compared with experimental values, showing that the proposed correlations could predict heat transfer coefficient well even neglected Schmidt number and Jacobi number.(3) The condensation enhancement inside CLFTs was studied experimentally by the method of thermal resistance separation. The heat transfer performance inside CLFTs was greater than that in smooth tube, not only for pure steam condensation but also for gas mixture condensation. During pure steam condensation, the enhancement of CLFTs decreased with the increase of vapor quality and mass flux. Both the correlation for heat transfer Nusselt number and correlation for pressure drop gradient were developed by fitting experimental data. The maximum performance evaluation factor was acquired in the minimum vapor quality and mass flux. The maximum value was 2.24 happened in a CLFT with pitch of 6 mm and depth of 0.7mm.(4) The heat transfer characteristic of steam/air mixture in CLFTs was researched. Compared with smooth tube, the decline range of heat transfer rate and coefficient owing to the increase of noncondensable gas mass fraction was smaller, and the increase range due to the increase of mass flux was larged. Moreover, the enhancement factor of heat transfer coefficient would increase with the increase of noncondensable gas mass fraction. For example, the enhancement factor increased from 1.99 to 2.31 when noncondensable gas mass fraction changed form 5% to 30%. But the improvement of enhancement factor for heat transfer rate was not obvious.(5) A theoretical model with non-uniform wall temperatures was developped for condensation in horizontal tubes, considering the change of mixture molecular weight, the suction effect caused by surface tension and the roughness effect caused by disturbance of gas-liquid interface. Through the numerical analysis, it was found that the thickness of liquid film gradually increased along the tube owing the increase of condensate and the thickness of gas film also increased because of the increase of noncondensable gas. The reason why the heat transfer coefficient enhancement factor increased with the increase of noncondensable gas mass fraction was revealed. For the higher noncondensable gas mass fraction, the condensation thermal resistance was controlled by liquld flim and gas flim thermal resistances. The spiral bulge in CLFTs could produce vortex in liquid and gas flims, destroying the stability of liquid and gas flims. Then both the heat transfer in liquid flim and mass transfer in gas flim were enhanced. |
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