Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
ES3C9 FLUID MECHANICS FOR MECHANICAL ENGINEERS Example Class 5 (1) A coastal power plant takes in cooling water through a vertical perforated manifold, as in figure 1. The total volume flow Q past the manifold, as shown. Estimate (a) how far downstream and (b) how far normal to the paper the effects of the intake are felt in the ambient 8m deep waters. Fig 1: Q 110 2.19m 2 s 1 (lecture 14) 2 b 16 m 2.19 8.75m (lecture 15) Then the stagnation point is at a U 0.25 And L 2 a 55m a (Stagnation line r , so when 0 and r then r sin a ) sin The sink strength is m (2) Effect of mirror design on the fuel consumption of a car As part of the continuing efforts to reduce the drag coefficient and thus to improve the fuel efficiency of cars, the design of side rear-view mirrors has changed drastically from a simple circular plate to a streamlined shape. Determine the amount of fuel and money saved per year as a result of replacing a 13cm diameter flat mirror (CD =1.1) by one with a hemispherical back (CD=0.4) (figure 2). Assume the car is driven L=24000 km a year at an average speed of 95km/h. Take the density and price of gasoline to be 0.8kg/L and £0.80/L, respectively; the heating value (HV) of gasoline to be 44000 kJ/kg and the overall efficiency of the engine to be ηcar =30%. The effect of the car body on the flow around the mirror is negligible (no interference). The air density is 1.2kg/m3. Fig 2: The drag force acting on a body is F CD A body. So for both mirrors, A V 2 2 , where A is the frontal area of the D2 . Then FD-flat =6.1 N. 4 Over one year, the work of the drag is W D-flat =F D-flat *L=146000kJ/year. The energy required for one year is : Ein= W D-flat / ηcar=488000kJ/year. m fuel Ein / HV 488000 / 44000 13.9 L / year Amount of fuel V fuel flat fuel fuel 0.8 Cost flat =13.9*0.8=£11.12 /year The drag and the work done to overcome are directly proportional to the drag coefficient. Then the percent reduction in the fuel consumption due to replacing the mirror is equal to the percent reduction in the drag coefficient: CD _ flat CD _ hemi 0.636 CD _ flat Fuel reduction = 0.636*13.9=8.84L/year Reduction ratio = Cost reduction = 0.636*11.12=£7.07/year Since a typical car has two mirrors, the driver saves more than £14 per year in gasoline by replacing the flat mirrors with hemispherical ones.