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MassSolutions

# MassSolutions - ME 201 Thermodynamics Conservation of Mass...

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1 ME 201 Thermodynamics Conservation of Mass Practice Problems 1. A human being can blow air out of their mouth at a rate of 10 -4 kg/s. How long will it take for this human to blow up a balloon to a volume of 5 x 10 -4 m 3 ? The air may be taken to be at 311 K and 110 kPa. Solution: This is a conservation of mass problem with one inflow and no outflows and zero initial mass. Writing the conservation of mass, we have in initial final m t m m & = - Assuming that the initial mass is zero and solving for the time period, we have m m t in final & = For the final mass, we have v V m final final = We will determine the specific volume, v, for air from the ideal gas equations, or /kg m 0.81 (110) 1) (0.287)(31 P RT v 3 = = = Then kg 10 x 6.17 0.81 10 x 5 m 4 - -4 final = = So that s 6.2 10 10 x 6.17 t 4 - -4 = = 2. Consider a 40 gallon hot water heater. Over a fifteen minute time period hot water flows out of the hot water heater at 0.75 kg/s and cold water at 0.5 kg/s flows into the hot water heater. How full (in percent) is the hot water heater at the end of fifteen minutes? You may take the water temperature to be 85ºC.

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MassSolutions - ME 201 Thermodynamics Conservation of Mass...

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