1、1Heat Conduction Analysis on the Evaporator in Gravity Heat Pipe of Heavy Oil WellboreAbstract: A technology for using petroleum deposits energy and the principle of mediums phase change heat transfer to make hollow rod into heat pipe，which transferred heat from bottom to top in wellbore by using it
2、 without extra energy is proposedIt can improve the temprature distribution of the fluid at the upper part of wellbore , therefore paraffin deposition and flocculation are improved. In this paper, heat transfer model of liquid film and liquid pool is established by means of the equation of N-S.Based
3、 on the principle of micro unit in liquid films thermal equilibrium and liquid pools heat transfer.By analying the heat transfer coeffcients of this two part,it was found out that gravity heat pipe had better heat transfer performance with increasing the length of liquid film in evaporator,improving
4、 the flow rate of inner steam and strengthening nucleate boiling of liquid pool,when the requirement of the continuous circulation of two-phase flow was achieved Keywords: gravity heat pipe; evaporator; heat conduction 2process; heat transfer coefficient 0 Introduction As the gravity heat pipe with
5、high efficiency, simple structure and low cost advantages,it is widely used in all kinds of heat transfer and heat recovery equipments1. At present,although two phase fluid flow and heat transfer inheat pipe get in-depth research, and has significant progress, but the study of the process of evapora
6、tion-condensation in super long gravity heat pipe which is made of hollow sucker rod is very little. Numerical simulation is an important method of studing the heat transfer performance of heat pipe, it provides a reliable prediction for various operating conditions of heat pipe. In this paper, we a
7、nalysis the thermal conductivity of gravity heat pipes evaporation in mathematical method, in order to provide theory basis for optimizing the gravity heat pipes evaporation parameters and improving heat pipes heat transfer performance. 1.Calculation model 1.1 Mathematical model of liquid membrane W
8、e make the process of thermal conductivity in gravity heat pipe evaporators liquid membrane into simplification and assumpt that: 3(1)The thickness of liquid film is far less than pipe diameter,?internal heat transfer is pure heat conduction, liquid membrane surface is smooth without fluctuation, th
9、e internal temperature is in linear distribution and ignore the convective heat transfer effects; (2)The state of gas is pure and saturation,the flow rate of interface gas is equal to the average flow velocity,?vapor density relative to the liquid density is neglected; (3)The temperature of the liqu
10、id film on the gas-liquid interface is equal to the saturation temperature,?ignore the viscous stress of interface steam against liquid film; (4)The working medium in heat pipe is often property; (5)Wall temperature distribute uniform. The basis of filmwise condensation2 theory solving is using the
11、flow boundary layer theory which is proposed by Prandtl3 and the concept of thermal boundary layer which is proposed by Pohlhausen4, simplify the Navier ? Stokes5 equation and then get the differential equation of thermal boundary layer heat transfer is as follows: In these formulas, , u, , g, T, ,
12、, yrepresent the dynamic viscosity, velocity, density, the acceleration of gravity, temperature, the shear stress of liquid film thickness 4and radial coordinate separately; , ,in subscript correspond to liquid, wall temperature of evaporation section, the interface of gas-liquid ,and the saturated
13、state. We can get the relation of velocity and the temperature distribution by integrating formula (4)and (5)on these two boundary conditions above: The relation of the velocity of liquid film ion the interface of gas-liquid uli,the mass flow rate of liquid film on the unit?width qm,the thickness of
14、 liquid film ,the drop height of liquid film X and the shear stress of interface i can be got by using the control equations above and boundary conditions: Cp, hfg, in the formula(13)represent specific?heat?capacity,?latent heat of vaporization and thermal?conductivity. In?the?formulas above,the flo
15、w velocity of liquid film on interface uli can be calculated byformula(10),using the gas mass balance on the same interface can get the average flow velocity of gas:uv=4qm/vd, The friction coefficient Cf is a function of gas-liquid phase to the Reynolds number. 1.2 mathematical model of liquid pool
16、In the process of heat transfer in fluid pool,the function 5of boiling has great influnce on the height of liquid pool and heat transfer coefficient?, in this paper, the function of bubble in liquid pool is taken intio account by introducing the experience formula of the coefficient of hollow bubble
17、 in drift model : In this formula,C0 is the distribution coefficient, which is a function of the density of gas-liquid,and the circular tube adopts the following empirical formula: Local show velocity of gas can be got by the balance equation of liquid pool: Dimensionless flowing velocity of gas Vvj
18、+ is a function of liquid viscosity Nf and dimensionless hydraulic diameter: When the height of Liquid pool dont beyond the height of evaporation section,the relation between the average cavitation coefficient of evaporation in liquid pool and the height of liquid pool Lp is as follows: 2. The resul
19、ts of numerical simulation in evaporation In this paper, using ammonia water as the working medium, the length of gravity heat pipe is 1000 meters, the evaporation section is 400 meters, liquid volume is 200 v/m, the operation pressure of the heat pipe is 2.39 MPa, and the pressure coefficient of st
20、ratum is 0.15, the temperature of groud is 20 6,the temperature gradient of stratum is 3/100m. (1)The thickness of liquid film The figure can be seen that the thickness of liquid film in evaporation become thin with the increase of the depth of the heat pipe,?and in this process,the thickness change
21、 of the liquid is relatively slow, the reasons for this phenomenon lies in the temperature difference of gravity heat pipe walls inside and outside is small, and the steam flow rate is faster, the flushing action against the liquid film is larger, thus a large number of droplets are upward with stea
22、m to make the liquid film into thin gradually, the speed of change will gradually speed up. (2)Flow velocity of steam and liquid film Figure 2 illustrates that the average flow velocity of steam is greater than the flow velocity of liquid film in the gas-liquid interface, steam flow is upward from t
23、he liquid pool to the junction of evaporator and condenser ,and the flow velocity raise gradually from 6.562 m/s to 9.728 m/s, this is due to the steam which from the liquid pool upward absorbs the heat of the steam unceasingly, the gas flow increase gradually, thus increasing speed. Because the car
24、rying capacity ofsteam in evaporation in power to carry on the role of the liquid film is 7greater than the effect of gravity on liquid film, it makes the liquid film from the top of the condenser section downward to the evaporation, the flow velocity decreases gradually,the flow velocity of liquid
25、film at the junction of evaporator and condenser is 0.261 m/s, and down to 0.203 m/s at liquid level. When the liquid film is laminar membrane, the thickness of liquid film is lesser, heat flow density is low, its heat transfer coefficient at the highest stage. With the increase of heat transfer rat
26、e, the thickness of liquid film is increasing, the heat transfer coefficient of the liquid film of evaporator in heat pipe will reduce with the increase of heat flux, before it into nuclear boiling.Because the effects of changes on liquid films thickness will recede after the liquid film completely
27、transfrom into nucleate boiling, the heat transfer coefficient will be increased with the increase of heat flux. The heat transfer coefficient of liquid pool is the lowest in natural convection , it will be increased gradually along with the transition of the mechanism of heat transfer and the incre
28、asing of heat flow density. 3 conclusion The heat transfer process of liquid film and liquid pool in the evaporation of heat pipe which is made of hollow sucker rod has a decisive effect on the heat transfer performance of heat 8pipe.In this paper,we establish mathematical model of the thermal condu
29、ctivity of liquid film and liquid pool in evaporation section, calculate to get the formula of the mass and heat transfer rate of the two, and get the change rule that their heat transfer rate?changed with the change of heat flow density by analyzing the calculation results, and analysis qualitative
30、ly that increase the length of the liquid film in evaporation, improve the steam flow rate inner and strengthen the nucleate boiling of liquid pool can improve the heat transfer performance of gravity heat pipe,in the condition of meeting the circulation of gas-liquid in the gravity heat pipe. Refer
31、ences 1Ma Tongze,Hou Zengqi,Wu Wenxian.HeatpipeM.Beijing:science publishing company.1983:4-8 2Yang Shiming,Tao Wenquan.Heat transmission science M.Beijing:Higher education publishing company.1998:331-335 3 El-Genk M S, Saber H H.Heat transfer correlations for liquid film in the evaporator of enclose
32、d, gravity-assisted thermosyphonsJ.Journalof Heat Transfer, Transactions ASME, 1998, 120( 2) : 477-484. 4 Kaminaga F,Okamoto Y,Suzuki T.Study on boiling heat transfer cor-relation in a closed two-phase 9thermosyphonC.Beijing:Proceeding of the 8th International Heat Pipe Conference.1992.317-322. 5 S.H.Noi.Heat transfer characteristics of a two-phase closed thermosypJ.Applied Thermal Engineering, 2005, 25: 495-506.
