Alright, let's dive into solving this problem!
a) The mass flow rate is calculated using the formula: mass_flow_rate = A1 * v1 / V1
Where A1 is the area of section 1, v1 is the velocity at section 1, and V1 is specific volume at section 1.
By substituting A1 with 20ft^3, v1 with 200 fpm and V1 with 2 ft^3/lb,
mass_flow_rate = 20ft^3 * 200 fpm / 2 ft^3/lb = 2000 lb/min
So, the mass flow rate is 2000 lb/min.
b) The speed at section 2 (v2) is calculated using the formula: v2 = mass_flow_rate * V1 / A2
Where mass_flow_rate is the rate calculated from part a, V1 is the specific volume at section 1, and A2 is the area of section 2.
By substituting mass_flow_rate with 2000 lb/min, V1 with 2 ft^3/lb and A2 with 4 ft^3,
v2 = 2000 lb/min * 2 ft^3/lb / 4 ft^3 = 1000 fpm
So, the speed at section 2 is 1000 fpm.
I hope these explanations are clear. If you have any questions, don't hesitate to ask!