When solving conservation of energy problems
A. Work on the system is added to the initial energy side
B. The change of thermal energy is zero if there is no friction or drag
C. Work is zero if the system is isolated
D. All of these are correct
Ans: D. All of these are correct
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if the extrusion cross-section is elliptical with the major axis normal to the air flow and same mass per unit length. The major axis of the elliptical cross-section is 5.46 cm and its perimeter is 12.8 cm
GIVEN
• A long elliptical copper extrusion in an air stream
• Initial temperature (To) = 400°C
• Air Temperature (T?) = 50°C
• Air velocity (V?) = 10 m/s
• Surface emissivity (?) = 0.9
• Elliptical cross-section with major axis normal to the air flow
• Length of the major axis of the ellipse (D) = 5.46 cm = 0.0546 m
• Perimeter of ellipse (P) = 12.8 cm = 0.128 m
• Same mass per unit length as Problem 6.9
FIND
• The time (t) required for the center of the copper to cool to 100°C
ASSUMPTIONS
• Air flow is perpendicular to the axis of the extrusion
• Variation of the copper properties with temperature is negligible
SKETCH
PROPERTIES AND CONSTANTS
the initial and final film temperature of 150°C
Thermal conductivity (ka) = 0.0339 W/(m K) Kinematic viscosity (?) = 29.6 × 10–69 m2/s Prandtl number (Pr) = 0.71
Thermal conductivity (k) = 386 W/(m K) at 250°C Density (?) = 8933 kg/m3 at 20°C Specific heat (c) = 383 J/(kg K) at 20°C