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Gases are often used instead of liquids to cool electronics in avionics applications because of weight considerations. The cooling systems are often closed so that coolants other than air may be used. Gases with high figures of merit (see Problem 6.30) are desired. For representative values of
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Fundamentals of Heat and Mass Transfer
- The capillary pressure for a channel of round cross section can be expressed as a function of the surface tension, the contact angle and the channel radius (the numerical factor is 2). Assume a transition of two coaxial channels of constant round cross sections. The larger one has a diameter of 2 mm and a contact angle of 80 degree, the narrower one a contact angle of 125 degree with the liquid. Calculate the other radius required so a liquid segment having one of its two menisci within either segment experiences a counter pressure of 15 mbar.arrow_forwardVolumetric thermal expansion is used in modeling natural convection heat transfer. Derive an expression for volumetric thermal expansion coefficient for an ideal gas.arrow_forwardWhen viscous dissipation is appreciable, the energy balance of a convective system must include the viscous effects of the fluid, so a third term is added to the energy balance: A slide bearing (pillow block) can be idealized as a static plate and a plane that moves parallel to it. The rotary axis of such a system has a diameter of 100 mm and rotates at 3600 rpm inside the bearing, which has a gap between the plates of 0.5 mm. The system is lubricated by oil, with a density of 800 kg / m, a viscosity of 0 01 kg / ms and a thermal conductivity of 0:14 W / mk. The top and bottom surfaces of the bearing are at 60 ° C. Considering the viscous effects of the oil, determine a) The maximum temperature of the oil. FC b) Bottom plate heat flux per unit length W / m Write down all assumptions madearrow_forward
- How is a lubricant drop be broken into finer droplets of a mist ?a. By increasing its kinetic energy by vibrating it at high frequencyb. By placing the drop into a zone of low pressure which is less than the internal pressure of the drop,generated by a venturi.c. By evaporating the drop into molecular form by reducing its pressure.d. By placing the drop into a highly turbulent zone such that a number of air jets blown onto it shearsit into fine droplets of a mist.arrow_forwardAir at atmospheric pressure and 25oC is heated in a tube with an ID of 25 mm. The tube wall is maintained at a temperature of 200oC. What length of tube is required to achieve an outlet air temperature of 120oC if the air flow is 10 Nm3/hr?arrow_forwardHeat Transfer with a Liquid Metal. The liquid metal bismuth at a flow rate of 2.00 kg/s enters a tube having an inside diameter of 35 mm at 425°C and is heated to 430°C in the tube. The tube wall is maintained at a temperature of 25°C above the liquid bulk temperature. Calculate the tube length required. The physical properties are as follows (H1): k = 15.6 W/m K, c,=149 J/kg K, u = 1.34 x 10-3 Pa s.arrow_forward
- In the final stages of production, a pharmaceutical is sterilized by heating it from 25 to 75°C as it moves at 0.22 m/s through a straight thin-walled stainless steel tube of 12.7-mm diameter. A uniform heat flux is maintained by an electric resistance heater wrapped around the outer surface of the tube. If the tube is 10 m long, what is the required heat flux? If fluid enters the tube with a fully developed velocity profile and a uniform temperature profile, what is the surface temperature at the tube exit? Fluid properties may be approximated as ρ=ρ= 1000 kg/m3, cp=cp= 4000 J/kg·K, μ=μ= 2 × 10-3 kg/s·m, k=k= 0.8 W/m·K, and Pr=Pr= 10.arrow_forwardWorking for an engineering consultancy firm, your knowledge of fluid dynamics is required to design a new safety feature for a high-pressure air line in a factory. The air line takes the form of a cylindrical pipe of diameter 150 mm, which is designed to operate between 0.45 MPa and 0.76 MPa. At the end of the pipe a bursting disk is placed so that, if the pressure exceeds the maximum operating pressure, the air is vented to atmosphere rather than over-pressuring the chemical reaction vessel (Figures 4a and 4b). In this question, you should treat the flow as quasione-dimensional and inviscid. The air in the surrounding atmosphere is at 101 kPa and 298 K. You have a choice of five disks which can withstand the following forces across them before bursting: 10.5 kN, 11.0 kN, 11.5 kN, 12.0 kN, 12.5 kN. Which of these bursting disks would you recommend, and why?arrow_forwardWorking for an engineering consultancy firm, your knowledge of fluid dynamics is required to design a new safety feature for a high-pressure air line in a factory. The air line takes the form of a cylindrical pipe of diameter 150 mm, which is designed to operate between 0.45 MPa and 0.76 MPa. At the end of the pipe a bursting disk is placed so that, if the pressure exceeds the maximum operating pressure, the air is vented to atmosphere rather than over-pressuring the chemical reaction vessel (Figures 4a and 4b). In this question, you should treat the flow as quasione-dimensional and inviscid. The air in the surrounding atmosphere is at 101 kPa and 298 K. Due to an over-pressurisation of the air line, the disk bursts at time t = 0. At what pressure in the air line will this occur?arrow_forward
- It is aimed to produce frozen potatoes in a newly established business. The entry temperature of the cleaned potato to the system is -16 °C, and the desired temperature to reach 10 °C. For this purpose, a refrigerant at -25 °C will be used. In the freezing process, potatoes of certain sizes are placed in the ice cream basket, the basket is immersed in the tank containing the refrigerant and kept during freezing. A cooling system is designed around the tank and 1M Joule per hour removes heat from the tank to keep the temperature of the tank constant at -25 °C. It is desirable to produce potatoes of different shapes. These shapes are spheres with a diameter of 20 mm, a cube with dimensions of 16x16x16 mm and a rectangular prism with a diameter of 12x12x25 mm. In such a process,a. What should be used as refrigerant?b. What is the daily production capacity for each product?c. What would you recommend to increase production capacity?arrow_forwardA copper rod of 30 kg initially at the temperature of 300 °C needs to be immersed and fitted tightly into a ABA alloy ring with inner diameter 30.0015 mm found in tank of 100 liters water of 25 °C. (i) Determine the diameter copper when it is 300 °C. (ii) In your opinion, does the copper rod will still fit tightly into the ABA alloy ring inner diameter 30.0015 in the ambient air condition of 25 °C. Provide your answer in 4 decimal places. Assume the alloy ABA is very small with negligible effect in heat transfer. [Density for copper = 8890 kg/m3, Specific heat, Ccopper = 385 J/kg.°C;Coefficient of thermal expansion, αCopper = 18.0 × 10-6 (°C)-1]. [Density for ABA alloy = 8500 kg/m3, Specific heat, CABA = 138 J/kg.°C;Coefficient of thermal expansion, αABA = 13.3 × 10-6 (°C)-1]. [Density for water = 1000 kg/m3, Specific heat, Cwater = 4200 J/kg.°C].arrow_forward
- Principles of Heat Transfer (Activate Learning wi...Mechanical EngineeringISBN:9781305387102Author:Kreith, Frank; Manglik, Raj M.Publisher:Cengage Learning