Question

A hair dryer has a fan and heating coils as shown. The steadly state mass flow rate through the hair dryer is given to be 15 g/s, and both inlet and exit pressures are standard atmospheric. If the exit temperature is 40°C warmer than the inlet temperature (15°C), answer the following (1) Calculate the inlet and velocities (averaged over respective surfaces). exit the Seerrey Calculate the horizontal force applied to the handle in order to hold the hair dryer stationary. A, = 25 cm (3) A, - 100 cm Assuming air to be a calorically perfect gas (Cos constant), what.net power must be supplied to the hair dryer (fan + heating)?

Answer 1

100cm 2 25 cm2 mass flow rate n = 159 1s B ha to follow the continuity equation mass flow rate must always be same throughout .. I a a S₂ Q₂ V2 = 5 S = 1.225 kg/m3, ₂ = 1.076 kg / m3 (from table ] ya in 15 x 10-3 Sia1 1.225 X 100 X 10-4 - 1.224 mis V2 - m - 15X10-3 3202 1.076 x 25 x 10-4 - 5.576 m/s 2. horizontal force a rate of change of momentum. = mv₂ - mv1 = 1.5x10 3 (5.57661 -1.124) - 6.678 x 10 -3 N Net power supplied to the hair dryer: - power supplied by coil + power delivered by fan.

29 it can also be seen by writing first haw for flow process. hq + v, 2 + g = hz + V ₂² + Ň wie hy h2 tvorevin Work is done by the pump so it is taken negative accourding to convention Net power supplied = hz-hi + V2 2 - 1 1 2 29 29 = cpl Tz -Ti) + V2 2 - u = 1:005 (40) +(5:522 12342; 1x69 29 2xo 2X9-

- - - 40 . 2 КТ | о 2 103 15 х 10s 603 vy т | s