Prominences hanging off the limb of the Sun are the same as filaments seen in front of the disk
Indicate whether the statement is true or false
TRUE
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Evaluate the dimensionless groups hcD/k, U? Dp/?, cp ?/k for water, n-Butyl alcohol, mercury, hydrogen, air, and saturated steam at a temperature of 100°C. Let D = 1 m, U? = 1 m/sec, and hc = 1 W/(m2 K).
GIVEN
D = 1 m
U? = 1 m/s
hc = 1 W/(m2 K)
FIND
The dimensionless groups
? hc D/k (Nusselt number)
? U? D p/? (Reynolds number)
? cp ?/k (Prandtl number)
PROPERTIES AND CONSTANTS
From Appendix 2, at 100°C
Two resistors of values 6.0 and 14.0 ? are connected in parallel. This combination in turn is hooked in series with a 2.0-? resistor. What is the overall resistance of this combination?
a. 1.4 ? b. 11 ? c. 6.2 ? d. 22 ? e. 4.2 ?
An iPod battery is rated at 3.7 volts and operates its iPod and earbuds with a direct current of 865 milliamps. What is the power rating of this iPod?
A. 234 Watts B. 3.7 Joules per second C. 3.2 Watts D. 16 Watts
A frozen-food company freezes its spinach by first compressing it into large slabs and then exposing the slab of spinach to a low-temperature cooling medium. The large slab of compressed spinach is initially at a uniform temperature of 21°C; it must be reduced to an average temperature over the entire slab of –34°C. The temperature at any part of the slab, however, must never drop below –51°C. The cooling medium which passes across both sides of the slab is at a constant temperature of –90°C. The following data may be used for the spinach: density = 80 kg/m3; thermal conductivity = 0.87 W/(m K); specific heat = 2100 J/(kg K). Present a detailed analysis outlining a method estimate the maximum thickness of the slab of spinach that can be safely cooled in 60 min.
GIVEN
• Large slabs of spinach are exposed to a low-temperature cooling medium
• Initial uniform temperature (To) = 21°C
• Average temperature must be reduced to –34°C
• The temperature at any part must never drop below –51°C
• Cooling medium temperature (T?) = –90°C
• Density of spinach (?) = 80 kg/m3
• Thermal conductivity (k) = 0.87 W/(m K)
• Specific heat (c) = 2100 J/(kg K)
FIND
• Present a detailed analysis outlining a method to estimate the maximum thickness of the slab of spinach that can be safely cooled in 60 min
ASSUMPTIONS
• One dimensional conduction through the slab
• Constant and uniform thermal
properties
• The average temperature within the slab is equal to the average of the center and surface temperatures
SKETCH
