Lab 实验传热Experiment of heat transfer.doc

Lab 4 Experiment of heat transferI. Objectives 1. Measure the overall heat-transfer coefficient K for a double-tube heat exchanger and the film heat-transfer coefficient for the inside of a circular pipe. 2. Measure the correlation between the Nusselt number Nu and the Reynolds number Re for heat transfer by forced convection in turbulent flow in a tube.II. PrincipleHeat transfer from a warmer fluid to a cooler fluid, usually through a solid wall separating the two fluids, is common in chemical engineering practice. 1. Determination of the overall heat transfer coefficient KIn the light of the equation of overall heat flow rate (1.15) Equation (1.15) can be written as (1.16)where Q = rate of heat transfer in the exchanger, W A = total area of heat transfer surface, m2= logarithmic mean temperature difference, The overall coefficient K can be determined if the values of Q, A and are measured or given.(1) Rate of heat transferIn this experiment, steam flows through the annular space between the outside and the inside pipe and air flows through the inside pipe. Hence, the overall energy balance equation can be obtained, neglecting the heat loss. (1.17)where qmh = mass flux of steam condensed, kg/sqmc = mass flux of air, kg/sr h = vaporization heat of water, J/kgcp c =specific heat of air at a constant pressure, J/(kgK)t1 =temperature of inlet air, t2 =temperature of outlet air, The rate of heat transfer in the exchanger Q is calculated according to the heat gained by air. The air flow rate is measured with an orifice meter and its mass flux is (1.18) (1.19)where qV = volumetric flow rate of air, m3/sC0 = orifice coefficient, here C0=0.855A0 = area of orifice, here d0=0.023 m = air density at the inlet of the orifice meter, kg/m30 = indicator density of differential pressure meter, kg/m3R = reading of differential pressure meter, mmAir density can be calculated according to the ideal gas law. (1.20)where pa =local atmospheric pressure, Pat = air temperature of the inlet of the orifice meter,R =gauge pressure of the inlet of the orifice meter, Pa(2) Logarithmic mean temperature difference, (1.21)where T = steam temperature, t1= temperature of inlet air, t2= temperature of outlet air, (3) Total area of heat transfer surface, AA=dL (1.22)where L= length of heat-transfer tube, md=outside diameter of tube, m2. Measurement the film heat-transfer coefficient, For the heat-transfer between air and steam, if the heat resistance of pipe wall and the fouling resistance are negligible, then the relationship of the overall coefficient of heat transfer and the film coefficient of heat transfer is expressed as (1.23)where 1=film coefficient of heat transfer from pipe wall to air, W/(m2K) 2 =film coefficient of heat transfer from steam to pipe wall, W/(m2K)For the film heat-transfer coefficient for the inside of the tube greater than that for the outside of the pipe for this experiment, the overall heat-transfer coefficient K approximately equals the film coefficient for the inside of the pipe. (1.24)3. The empirical relation of Nu and ReBased on dimensional analysis, the film heat-transfer coefficient of air for turbulent flow inside the tube conforms to the following equation. or (1.25)where Nu = Nusselt numberRe = Reynolds number= air thermal conductivity, W/(mK)u = air flow velocity, m/s= air density, kg/m3= air viscosity, kg/(ms)A, n=coefficients to be determinedIn this experiment, the film heat-transfer coefficient can be measured by adjusting the air flow rate. Then, plot a Nu-Re curve on a double logarithmic paper and to determine coefficients of A and n. An empirical equation of the relationship between the film heat-transfer coefficient and Re can be obtained. III. ApparatusThe setup is consisted of two double-pipe exchangers. The inside pipe for one is made from a smooth brass pipe, and that for the other is made from a screwed slot brass pipe. The outside pipe for both exchangers is steel pipe.Fig. 1.6 Heat transfer in a double-pipe heat exchanger between vapor and air1air blower; 2orifice flow meter; 3air regulating valve; 4double pipe heat exchanger; 5viewing mirror; 6thermometer; 7thermal couple; 8safe valve; 9steam pressure gauge; 10manometer; 11steam inlet valve; 12blow-down cock; 13steam trap; 14alternation switch of thermal couple; 15differential electrometer; 16ice potIV. Experimental procedures1. Prime the tap water into the water tank of the steam generator. When the water level reaches the red line of water level gauge, switch on the power, and open the air discharge valve to drain non-condensable gases.2. When the water is boiling, switch off one or two sets of heater, then regulate the temperature controller to a given value.（normally the temperature is less than 135）. 3. Check the above steps and proceed.4. During the period of operation，the water level must be between upper and lower red lines. If the water level is below the red line, switch off the power, open the air discharge valve, and refill water.5. When the steam pressure reaches a certain value (0.1MPa), start the air blower, open steam inlet valve, and regulate the air flowrate. Measure 6 to 8 sets of data for suitable flowrate intervals. During the operation, open the air discharge valve regularly to drain the non-condensable gases.6. Switch off the power and the differential electrometer, open the air discharge valve, and stop the air blower.V. Cautions 1. Control the stream pressure not to exceed the pressure limit. Observe the water level of the water tank.2. During the experiment, the joint point of the thermal couple should be placed in an ice pot to keep it around zero degree.VI. Data recordsoutside diameter of pipe： ; pipe length： m; flow coefficient： ; room temperature： Table 1.4 Data records for heat transfer experimentNumberAir flow rateGauge pressure Temperature R/mmqV/m3.s-1P表Steam Air inletAir outletmmPamVmVmV12345678VII. Report object