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Thumb for Mechanical Engineers 2011 Part 3

Tham khảo tài liệu 'thumb for mechanical engineers 2011 part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Rules of Thumb for Mechanical Engineers ty-ÍPról 05 s 2500 where AP 0 is the frictional pressure drop for the liquid flow alone in the same tube with a mass flow rate equal to the total mass flow rate of the two-phase flow. The Martinelli-Nelson experimental curves of J lo vs. X show breaks in the slope due to changes in flow regimes. Surface tension is not included although it may have a significant influence at high pressure near the critical point. The Martinelli-Nelson method provides more correct results than the homogeneous model for low mass velocities G 1 360 kg m2s . In contrast the homogeneous model provides better results for high mass velocities. Chisholm gives the following correlation for flow of evaporating two-phase mixtures that accounts for some of the effects neglected in other methods 4 1 f2 - 1 Bx 2 n 2 1 - x 2 n 2 x2-n 18 Table 12 Values of B for Two-Phase Frictional Pressure-Drop Evaluation in Crossflow and Window-Flow Zones by Equation 18 Zone Horizontal Vertical Up and Down Flow Crossflow Spray and bubble Stratified and Stratified spray Window n 0 Sources 1. Cheremisinoff N. p. Heat Transfer Pocket Handbook. Houston Gulf Publishing Co. 1984. 2. Lockhart R. w. and Martinelli R. c. in Chem. Engrg. Prog. 45 39 18 1949. 3. Martinelli R. c. and Nelson D. B. in Transactions ASME 70 695 1948. 4. Chisholm D. Inti. Joum. of Heat and Mass Transfer 16 347-358 1973. APf AP o n is the exponent in the Blasius relation for friction factor f C Ren with n 0.25 for the turbulent flow. These discussions are inclusive of tube flow only. Two-phase pressure-drop correlations for the shell-side flow are available for a segmentally baffled shell-and-tube exchanger. The frictional pressure drop consists of two components one associated with the crossflow zone and the other with the window zone. Grant and Chisholm determined the components of the pressure drop 4 The two-phase crossflow zone and window zone frictional pressure drops are given by Equation 18 with .