The temperature of a flowing gas is to be measured with a thermocouple junction and wire stretched between two legs of a sting, a wind tunnel test fixture. The junction is formed by butt-welding two wires of different material, as shown in the schematic. For wires of diameter D = 125 μm and a convection coefficient of h = 700 W/m 2 ⋅ K, determine the minimum separationdistance between the two legs of the sting, L = L 1 + L 2 , to ensure that the sting temperature does not influence the junction temperature and, in turn. invalidate the gas temperature measurement. Consider two different types of thermocouple junctions consisting of (i) copper and constantan wires and (ii) chromel and alumel wires. Evaluate the thermal conductivity of copper and constantan at T = 300 K . Use k Ch = 19 W/m ⋅ K and k A1 = 29 W/m ⋅ K for the thermal conductivities of the chromel and alumel wires. respectively.
The temperature of a flowing gas is to be measured with a thermocouple junction and wire stretched between two legs of a sting, a wind tunnel test fixture. The junction is formed by butt-welding two wires of different material, as shown in the schematic. For wires of diameter D = 125 μm and a convection coefficient of h = 700 W/m 2 ⋅ K, determine the minimum separationdistance between the two legs of the sting, L = L 1 + L 2 , to ensure that the sting temperature does not influence the junction temperature and, in turn. invalidate the gas temperature measurement. Consider two different types of thermocouple junctions consisting of (i) copper and constantan wires and (ii) chromel and alumel wires. Evaluate the thermal conductivity of copper and constantan at T = 300 K . Use k Ch = 19 W/m ⋅ K and k A1 = 29 W/m ⋅ K for the thermal conductivities of the chromel and alumel wires. respectively.
Solution Summary: The author calculates the minimum separation distance between two legs of the sting. The thermal conductivity of Chromel is k_ch=19W/m
The temperature of a flowing gas is to be measured with a thermocouple junction and wire stretched between two legs of a sting, a wind tunnel test fixture. The junction is formed by butt-welding two wires of different material, as shown in the schematic. For wires of diameter
D
=
125
μm
and a convection coefficient of
h
=
700
W/m
2
⋅
K,
determine the minimum separationdistance between the two legs of the sting,
L
=
L
1
+
L
2
,
to ensure that the sting temperature does not influence the junction temperature and, in turn. invalidate the gas temperature measurement. Consider two different types of thermocouple junctions consisting of (i) copper and constantan wires and (ii) chromel and alumel wires. Evaluate the thermal conductivity of copper and constantan at
T
=
300
K
.
Use
k
Ch
=
19
W/m
⋅
K
and
k
A1
=
29
W/m
⋅
K
for the thermal conductivities of the chromel and alumel wires. respectively.
1. Saturated steam at 500 K flows in a 0.20 m inside diameter, 0.21 m outside
diameter pipe. The pipe is covered with 0.08 m of insulation with a thermal
conductivity of 0.10 W/m-K. The pipe's conductivity is 52 W/m-K. The ambient
temperature is 300 K. The unit convective coefficients are h; = 18,000 W/m²-K and
ho = 12 W/m²-K. Determine the heat loss (kJ/min) from 4 m of pipe.
• show conversions, units, and box in your final answers
t = 30 + 0.9563 (62.2- 30) = 60.79°C (Ans.)
Example 4.14. A very thin glass walled 3 mm diameter mercury thermometer is placed in a
stream of air, where heat transfer coefficient is 55 W/m2°C, for measuring the unsteady temperature
of air. Consider cylindrical thermometer bulb to consist of mercury only for which k
and a = 0.0166 m2/h. Calculate the time required for the temperature change to reach half its final
or,
%3D
8.8 W/m C
%3D
value.
A thermometric well is placed in a pipe having diameter of
55 mm. Pipe wall temperature is 100°C and heat transfer
coefficient inside the pipe is 300 W/m² K. Thickness of
thermowell is 1.2 mm and its length is 50 mm. Thermal
conductivity of thermowell material. Is 30 W/mK. If the
temperature of the gas flowing through the pipe is recorded
by thermometer as 200°C, determine the true temperature of
gas. If the error in gas temperature is to be reduced by 80%
by increasing the length of thermowell, determine the new
length of thermowell. Draw a sketch of this thermowell. Take
perimeter to area ratio for thermowell as 1/T (T= thickness).
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