Research On Gas-liquid Film Thicknesses And Heat Transfer Characteristics Of Vapor-Gas Condensation

Well known, the phenomena of vapor-gas condensation widely exist in heat transfer processes in industries and its heat transfer sharply deteriorates in the presence of non-condensable gas. It is therefore significant to study vapor-gas condensation heat transfer and improve its heat transfer efficiency for economic benefits and energy saving.Generally speaking, the heat resistances of vapor-gas condensation heat transfer mainly include gas phase heat resistance caused by non-condensable gas, liquid film heat resistance due to the condensate. Among them, gas phase heat resistance is the uppermost factor that makes condensation heat transfer deteriorated, and liquid film heat resistance is also harmful to its efficiency. According to previous research, they are determined by gas film thickness and liquid film thickness, respectively, dependent on tube geometry, the arrangement, flow velocity, flow direction, thermal and physical parameters, etc. In view of those above, the thesis is to primarily investigate gas liquid film thicknesses and heat transfer characteristics of vapor-gas condensation outside a horizontal tube by the methods of theoretical analysis and numerical calculation, in order to make a foundation for its further research and application.After some efforts, this thesis developed the mathematical models of gas liquid film thicknesses and heat transfer characteristics of vapor-gas condensation outside a horizontal tube. Definitely, it is concluded that:First, for free convection, gas liquid film don’t separate for a horizontal circular tube, whereas liquid film may separate but gas film never does for a horizontal non-circular tube. Second, for forced convection, three different cases have been demonstrated for different values of bulk velocity, namely, both film non-separating, only gas film separating yet liquid film non-separating, and liquid film separating with gas film already having separated. In addition, for a circular tube, the critical velocity of gas film separation is Um=Jgd/8, and its separation angle is θm=arcos(gd/8U2). The critical velocity of liquid film separation is U1=(?) and its separation angle is θ1=arcos[-(ρ1-ρ)gd/(8ρU2)] Third, the condensation heat transfer is weak because of great gas phase heat resistance before gas film separating. However, it is enhanced somehow after gas film separates and it is more greatly increased after liquid film separating. Forth, gas film thickness sharply increases near the bottom of the tube or the separation point for free convection and gas film separating for forced convection. Yet gas film thickness rapidly decreases near the bottom of the tube for both film non-separating for forced convection. Fifth, for forced convection, gas film thickness increases with flow velocity increased if gas film separates, while gas film thickness decreases with flow velocity increased if gas film doesn’t separate. Sixth, the heat transfer rate increases with pressure increased, and does with non-condensable gas concentration or tube wall temperature decreased. What’s more, along the tube surface does it first decrease and backward increase with flow velocity increased. Seventh, for tubes with identical geometry, gas liquid film thicknesses decrease with the curvature increased and the separating area increases with the curvature increased when liquid separates. So the overall heat transfer rate increases with the curvature increased. But for different tube geometry, it need thoroughly consider all the factors for the overall heat transfer rate, such as gas liquid film thicknesses and the separating area. Eighth, the discharge and separation of gas film associates with parameter A, and those of are related with liquid film parameter B.For the waste heat recovery of flue gas in fuel oil and fuel gas boilers, the study acquires the distributions of gas liquid film and heat transfer characteristics in different operating condition using the model of gas liquid film thicknesses. Moreover, the heat transfer characteristics with and without liquid film heat resistance are compared by calculating. Consequently, the design value of the heat transfer area is smaller than the true value ignoring the heat resistance of liquid film, while it is close to the true value considering that heat resistance in the design of heat exchanger. As a result, it is useful for the distribution characteristics of gas liquid film to provide a certain reference to design the best heat exchange device.