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1.
An un-insulated air conditioning duct of rectangular cross section 1 m × 0.5 m, carrying air at 20°C with a velocity of 10 m/s, is exposed to an ambient of 30°C. Neglect the effect of duct construction material. For air in the range of 20-30°C, data are as follows: thermal conductivity =0.025 W/m.K; viscosity = 18:Pa.s; Prandtl number =0.73; density = 1.2 kg/m3. The laminar flow Nusselt number is 3.4 for constant wall temperature conditions and, for turbulent flow, Nu=0.023 Re0.8 Pr0.33 .
[1] The Reynolds number for the flow is [2 marks]
(a) 444
(b) 890
(c) 4.44×105
(d) 5.33×105[2] The heat transfer per metre length of the duct, in watts, is: [2 marks]
(a) 3.8
(b) 5.3
(c) 89
(d) 769asked in Mechanical Engineering, 2005
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2.
The following table of properties was printed out for saturated liquid and saturated vapour of ammonia. The titles for only the first two columns are available. All that we know is that the other columns (columns 3 to 8) contain data on specific properties, namely, internal energy (kJ/kg), enthalpy (kJ/kg) and entropy (kJ/kg.K).
[1] The specific enthalpy data are in columns [2 marks]
(a) 3 and 7
(b) 3 and 8
(c) 5 and 7
(d) 5 and 8[2] When saturated liquid at 40°C is throttled to –20°C, the quality at exit will be
[2 marks]
(a) 0.189
(b) 0.212
(c) 0.231
(d) 0.788asked in Mechanical Engineering, 2005
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3.
Consider a linear programming problem with two variables and two constraints. The objective function is:
Maximize X1 + X2.
The corner points of the feasible region are (0,0), (0,2), (2,0) and (4/3, 4/3).
[1] If an additional constraint X1+ X2 ≤ 5 is added, the optimal solution is [2 marks]
(a) (5/3, 5/3)
(b) (4/3, 4/3)
(c) (5/2, 5/2)
(d) (5, 0)
[2] Let Y1 and Y2 be the decision variables of the dual and v1 and v2 be the slack variables of the dual of the given linear programming problem. The optimum dual variables are
[2 marks]
(a) Y1 and Y2
(b) Y1 and v1
(c) Y1 and v2
(d) v1 and v2asked in Mechanical Engineering, 2005
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4.
A band brake consists of a lever attached to one end of the band. The other end of the band is fixed to the ground. The wheel has a radius of 200 mm and the wrap angle of the band is 270°. The braking force applied to the lever is limited to 100N, and the coefficient of friction between the band and the wheel is 0.5. No other information is given.
[1] The maximum tension that can be generated in the band during braking is [2 marks]
(a) 1200 N
(b) 2110 N
(c) 3224 N
(d) 4420 N[2] The maximum wheel torque that can be completely braked is [2 marks]
(a) 200 N.m
(b) 382 N.m
(c) 604 N.m
(d) 844 N.m
asked in Mechanical Engineering, 2005
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5.
The complete solution for the ordinary differential equation
d2y/dx2 + pdy/dx +qy = 0 is y= c1e-x +c2e-3x
[1] Then, p and q are [2 marks]
(a) p = 3, q = 3
(b) p = 3, q = 4
(c) p = 4, q = 3
(d) p = 4, q = 4[2] Which of the following is a solution of the differential equation
d2y/dx2 + pdy/dx + (q+1)y = 0 [2 marks]
(a) e-3x
(b) xe-x
(c) xe-2x
(d) x2e-2x
asked in Mechanical Engineering, 2005
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6.
In two air standard cycles – one operating on the Otto and the other on the Brayton cycle – air is isentropically compressed from 300 to 450 K. heat is added to raise the temperature to 600 K in the Otto cycle and to 550 K in the Brayton cycle.
[1] If no and nB are the efficiencies of the Otto and Brayton cycles, then [2 marks]
(a) no=0.25, nB =0.18
(b) no= nB = 0.33
(c) no= 0.5, nB =0.45
(d) it is not possible to calculate the efficiencies unless the temperature after the expansion is given.
[2] If Wo and WB are work outputs per unit mass, then [2 marks]
(a) Wo > WB
(b) Wo < WB
(c) Wo = WB
(d) it is not possible to calculate the work outputs unless the temperature after the expansion is given.asked in Mechanical Engineering, 2005
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