heat transfer1adi for mechanicxal emgineering student.pptx

heat transfer ppt

1. Title: R ADIATION Presented by: Aditya Singh ( UnivRollNo .- 14200720008 ) Subject Name~ Heat Transfer 3-Aug-22 Department of Mechanical Engineering 1

2. Contents: Radiation Objective of the HT analysis Problem Statement Schematic Assumptions Analysis Comments 3-Aug-22 Heat Transfer Activity-1 2

3. Radiation All physical matter emits thermal radiation in the form of electromagnetic waves because of vibrational and Rotational movements of the molecules and atoms make up the matter. Objective of the HT analysis 3-Aug-22 Heat Transfer Activity-1 3

4. Problem Statement Chips of width = 15 mm on a side are mounted to a substrate that is installed in an enclosure whose walls and air are maintained at a temperature of Tsur =T ∞ = 25°C. The chips have an emissivity of = 0.60 and a maximum allowable temperature of Ts= 85°C. If heat is rejected from the chips by radiation and natural convection, what is the maximum operating power of each chip? The convection coefficient depends on the chip-to-air temperature difference and may be approximated as h = C(Ts - T ∞ .)^1/4, where C= 4.2 W/m².K^5/4. b. If a fan is used to maintain air flow through the enclosure and heat transfer is by forced convection, with h =250 W/m^2.k, what is the maximum operating power? 3-Aug-22 Heat Transfer Activity-1 4

5. Known Width, surface emissivity and maximum allowable temperature of an electronic chip. Temperature of air and surroundings. Convection coefficient. FIND: (a) Maximum power dissipation for free convection with h(W/m^ 2 ⋅ K) = 4.2(T - T ∞ )^1/4, (b) maximum power dissipation for forced convection with h = 250 W/m^ 2 ⋅ K. 3-Aug-22 Heat Transfer Activity-1 5

6. Schematic 3-Aug-22 Heat Transfer Activity-1 6

7. Assumptions Steady-state conditions (2) Radiation exchange between a small surface and a large enclosure (3) Negligible heat transfer from sides of chip or from back of chip by conduction through the substrate. 3-Aug-22 Heat Transfer Activity-1 7

8. Analysis 3-Aug-22 Heat Transfer Activity-1 8

9. Comments Clearly, radiation and natural convection are inefficient mechanisms for transferring heat from the chip. For Ts = 85 ° C and T ∞ = 25 ° C, the natural convection coefficient is 11.7 W/m 2 ⋅ K. Even for forced convection with h = 250 W /m2 ⋅ K, the power dissipation is well below that associated with many of today’s processors. To provide acceptable cooling, it is often necessary to attach the chip to a highly conducting substrate and to thereby provide an additional heat transfer mechanism due to conduction from the back surface. 3-Aug-22 Heat Transfer Activity-1 9