Heat Transfer T...

The temperature variation under steady heat conduction across a composite slab of two materials of conductivities k₁ and k₂ is shown in Fig. Then which one of the following statements holds?

Two finned surfaces with long fins are identical, except that the convention heat transfer coefficient for the first finned surface is twice that of second one. Then the correct statement for the efficiency and effectiveness of the first finned surface relative to second one.

A metal sphere of surface area 0.0225 m² is an evacuated enclosure whose walls are held at very low temperature. Electric current is passed through resistors embedded in the sphere causing electrical energy to be dissipated at rate of 75 watts. If sphere surface temperature is measured to be 560 K while is steady state. The reflectivity of sphere is

A wall of thickness 0.6 m has width has a normal area 1.5 m² and is made up of material of thermal conductivity 0.4 W/mK. The temperatures on the two sides are 800°C. What is the thermal resistance of the wall?

A composite wall of a furnace has 2 layers of equal thickness having thermal conductivities in the ratio of 3 : 2. What is the ratio of the temperature drop across the two layers?

A fluid (Prandtl number, Pr = 0.7) at 500 K flows over a flat plate 500 mm length, maintained at 300 K. The velocity of the fluid is 10 m/s. Assuming kinematic viscosity = \(30 \times {10^{ - 6}}{m^2}/s\), then which of the following statements are true regarding the boundary layer thickness at 5 mm from the leading edge?

The effectiveness of a counter-flow heat exchanger has been estimated as 0.25. Hot gases enter at 200°C and leave at 75°C. Cooling air enters at 40°C. The temperature of the air leaving the unit will be:

One dimensional unsteady state heat transfer equation for a sphere with heat generation at the rate of 'q' can be written as

For the three-dimensional object shown in thefigure below, five faces are insulated. Thesixth face (PQRS), which is not insulated,interacts thermally with the ambient, with aconvective heat transfer coefficient of 10 W/m².K. The ambient temperature is 30°C. Heatis uniformly generated inside the object at therate of 100 W/m³. Assuming the face PQRS tobe at uniform temperature, its steady statetemperature is:

Air at 1 atmospheric pressure and 27 °C blows across a 12 mm diameter sphere at a a small heater inside the sphere maintains the surface temperature at 77 °C. With k = 0.026 W/m (kelvin) and with (Nu) = 31.4, the heat loss by the sphere would be

A sphere with radius 20 cm has density 2500 kg/m³ and specific heat of 0.52 kJ/kg K has a uniform heat generation rate of 100 kW/m³. If heat received over it’s surface in 240 W, the rate of change of temperature of solid is ______ (°C/sec)

An air stream flows with a velocity of 3 m/s. The temperature of which is to be measured by a thermocouple which can be approximated as a sphere of diameter 0.25 cm. Initially the junction and air are at a temperature of 25°C. The air temperature suddenly changes and is maintained at 215°C. The time required for the thermocouple to indicate a temperature of 165°C is:

A bar of Cu of length 75 cm and a steel bar of 125 cm length are joined together end to end. Both the bars are having same diameter of 2 cm. The free ends of the Cu and the steel bars are maintained at 100°C and 0°C respectively. The curved surfaces of the bars are thermally insulated, what is the approximate temperature at Cu-steel junction? (consider conductivity k of Cu = 400 Js^-1m^-1 C^-1 and k for steel 50 Js^-1m^-1 C^-1)

A steel ball of mass 1kg and specific heat 0.4 kJ/kg is at a temperature of 60°C. It is dropped into 1kg water at 20°C. The final steady state temperature of water is:

A fin has 5 mm diameter and 100 mm length with insulated tip. The thermal conductivity of fin material is 400 Wm^-1K^-1. One end of the fin is maintained at high temperature and its remaining surface is exposed to ambient air. If the convective heat transfer coefficient is 40 Wm^-2K^-1, then

A long 15 cm diameter shaft of stainless steel (K = 14.9 W/mK), ρ = 7900 kg/m³ C = 477 J/kgK and α = 3.95 × 10^-6 m²/s is at a uniform temperature of 450 °C. The shaft is allowed to cool slowly in a chamber at 150°C with heat transfer coefficient of h = 85 W/m²K. Calculate the temperature (℃) at the end of 25 min after the cooling starts.

A large concrete slab 1 m thick has one dimensional temperature distribution:

T = 4 – 10x + 20x² + 10x³

Where T is temperature and x is distance from one face towards other face of wall. If the slab material has thermal diffusivity of 2 × 10^-3 m2/hr,what is the rate of change of temperature at the other face of the wall?

A cylindrical insulated pipe of diameter 0.3m has a surface temperature of 330K and is exposed to surrounding s at 298K. The emissivity and absorptivity of the pipe material are 0.89 and 0.92 respectively. If the surrounding are assumed to be transparent, find the % contribution of radiation to total heat transfer rate to surroundings. (Take convective heat transfer coefficient = 4.5 W/m²K)

Liquid metal flows through a circular tube in which the velocity and temperature profiles are governed by:

\(U\left( r \right) = {\rm{A}};\;T\left( r \right) - {T_W} = B\left[ {1 - {{\left( {\frac{r}{R}} \right)}^2}} \right]\)

A fluid (k = 0.6 W/m°C and ν = 10^-5 m²/s) enters a 10mm diameter and 15 m long tube at 298 K with a velocity of 0.5 m/s and leave the tube at 330 K. The tube is subjected to a uniform heat flux of 3 kW/m² on its surface. Calculate the temperature of the surface at the exit in (K)

Thermal diffusivity of a substance is:

A furnace is made of a red brick wall of thickness 0.5 m and conductivity 0.7 W/mK. For the same heat loss and temperature drop,this can be replaced by a layer of diatomite earth of conductivity 0.14W/mK and thickness

A plate of dimensions 35 cm × 15cm (width) is kept vertically. The plate is initially at a temperature of 413 K and the ambient temperature may be taken as 293K. Calculate the maximum possible rate of heat dissipation (W) from both sides neglecting any radiation effects and use the following data for reference.

A long rod 20 mm in diameter has one end maintained at 110°C. The surface of the rod is exposed to ambient air at 20°C with convection coefficient of 5 W/m²k. Calculate the heat loss (in W) from the rod having thermal conductivity of 15 W/mK.