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ME331: Introduction to Heat Transfer Spring 2017
ME 331 Homework Assignment #6
Problem 1
Statement: Water at 30oC flows through a long 1.85 cm diameter tube at a mass flow rate of 0.020
kg/s.
Find: The mean velocity (๐๐), maximum velocity (๐๐ด๐จ๐ฟ), and the pressure difference (๐ซ๐) between
two sections ๐ซ๐ = 150 m apart in the tube.
Schematic:
Assumptions:
1. Incompressible fully developed flow.
2. Constant properties for water at 30oC = 303.15K. Interpolating from Table A.6,
๐ = 995.8 ๐๐/๐3 , ๐ = 8.01 ร 10โ4 ๐ โ ๐ /๐2
Analysis:
From the mass flow rate, density, and cross sectional area, the mean velocity is found using eq. 8.5:
๐ข๐ =๏ฟฝฬ๏ฟฝ
๐๐ด๐=
๏ฟฝฬ๏ฟฝ๐
4๐ท2๐
=0.020
๐๐
๐ ๐
4(0.0185 ๐)2โ995.8
๐๐
๐3
= 0.074721๐
๐
๐ข๐ = 7.5 ร 10โ2 ๐
๐ ANSWER
Determine whether the flow is laminar or turbulent.
๐ ๐๐ท =๐๐ข๐๐ท
๐=
995.8๐๐
๐3โ0.074721๐
๐ โ0.0185 ๐
8.01ร10โ4 ๐โ๐ /๐2 = 1,718.8
The Reynolds number is less than the critical number of 2300; thus the flow is laminar. The velocity
profile is given by equation 8.15:
๐ท = 1.85 ๐๐
๐ฟ = 150 ๐
๐ = 303.15 ๐พ
๏ฟฝฬ๏ฟฝ = 0.020๐๐
๐
ME331: Introduction to Heat Transfer Spring 2017
๐ข(๐) = 2 ๐ข๐ [1 โ (๐
๐๐)
2
]
The maximum velocity at r = 0 is:
๐ข๐๐ด๐ = ๐ข(๐ = 0) = 2 ๐ข๐ = 0.1494 ๐/๐
๐ข๐๐ด๐ = 0.15๐
๐ ANSWER
The pressure gradient is related to the mean velocity by equation 8.14:
๐ข๐ = โ1
8๐(
๐๐
๐๐ฅ) ๐๐
2
For fully developed laminar flow, the pressure gradient is constant:
๐๐
๐๐ฅ= โ
8๐๐ข๐
๐๐2 = โ
8 โ 8.01 ร 10โ4 ๐ โ๐
๐2 โ0.074721๐
๐
(0.00925 ๐)2= โ5.5948 ๐๐/๐
The pressure drop over 150 m length of pipe is:
|ฮ๐| = |๐๐
๐๐ฅ| ๐ฟ = 5.5948
๐๐
๐โ 150 ๐ = 839.2194 ๐๐
|ฮ๐| = 840 ๐๐ ANSWER
ME331: Introduction to Heat Transfer Spring 2017
Problem 2
Statement: You want to heat water from 12oC to 70oC as it flows through a 2.0 cm internal diameter,
7.0 m long tube. The tube is equipped with an electric resistance heater, which provides uniform
heating at the outer surface of the tube. The heater is well insulated, so that in steady operation all
the thermal energy generated in the heater is transferred to the water in the tube. You want the
system to provide hot water at a rate of 8.0 L/min.
Find: The required power rating of the resistance heater, and the inner surface temperature at the
outlet (๐ป๐,๐).
Schematic:
Assumptions:
1. Steady flow conditions.
2. Uniform surface heat flux.
3. Inner surface of the tube is smooth.
4. Properties evaluated at the mean fluid temperature of (70+12)/2 = 41oC:
๐ = 992.1 ๐๐/๐3
๐ = 0.631 ๐/๐๐พ
๐ =๐
๐= 0.658 ร 10โ6 ๐2/๐
๐ = 4179 ๐ฝ/๐๐๐พ
๐๐ = 4.32
Analysis:
The mass flow rate through the tube and the required power rating of the resistance heater are:
๏ฟฝฬ๏ฟฝ = ๐๏ฟฝฬ๏ฟฝ = (992.1 ๐๐/๐3)(0.008 ๐3/๐๐๐) = 7.937 ๐๐/๐๐๐ = 0.132 ๐๐/๐
๐ = ๏ฟฝฬ๏ฟฝ ๐ (๐๐,๐ โ ๐๐,๐) = (0.132๐๐
๐ ) (4179
๐ฝ
๐๐๐พ) (70 โ 12)๐พ = 32.06 ๐๐
๐ = 32 ๐๐ ANSWER
๐ฟ = 7 ๐
๐๐,๐โฌ = 70ยฐ๐ถ
๐ท = 2 ๐๐
๐๐,๐โฌ = 12ยฐ๐ถ
๐" = ๐๐๐๐ ๐ก๐๐๐ก
๏ฟฝฬ๏ฟฝ = 8.0 ๐ฟ/๐๐๐
ME331: Introduction to Heat Transfer Spring 2017
To determine the inner surface temperature at the tube outlet we first determine whether the flow is
laminar or turbulent.
๐ข๐ =๏ฟฝฬ๏ฟฝ
๐ด๐=
(8
60ร10โ3)
๐
๐
3
๐(0.02 ๐)2
4
= 0.4244๐
๐
๐ ๐ =๐ข๐๐ท
๐=
(0.4244๐
๐ )(0.02 ๐)
0.658ร10โ6๐2/๐ = 12,890
This value is greater than the critical Reynolds number of 2300 for pipe flow. Therefore, the flow is
turbulent and the entry length is estimated at 10 pipe diameters:
๐ฅ๐๐,๐ก โ 10๐ท = 10(0.02 ๐) = 0.2 ๐
Since the entry length is much shorter than the total tube length, the thermal and hydrodynamic profiles
of the turbulent flow are assumed to be fully developed at L = 7 m. The local Nusselt number is
determined from:
๐๐ข =โ๐ท
๐= 0.023๐ ๐0.8๐๐0.3 = 0.023(12890)0.8(4.32)0.4 = 80.2
The local heat transfer coefficient is:
โ =๐
๐ท๐๐ข =
0.631๐
๐๐พ0.02 ๐
(80.2) = 2530 ๐/๐2๐พ
Using q = โ๐ด(๐๐ ,๐ โ ๐๐,๐), the inner surface temperature of the tube at the outlet is:
๐๐ ,๐ =๐
โ๐ด+ ๐๐,๐ =
32,060 ๐
2530๐
๐2๐พโ๐(0.02 ๐)(7 ๐)
+ 70ยฐ๐ถ = 98.8ยฐ๐ถ
๐๐ ,๐ = 99ยฐ๐ถ ANSWER
ME331: Introduction to Heat Transfer Spring 2017
Problem 3
Statement: A typical automotive exhaust pipe carries exhaust gas at a mean velocity of 6.0 m/s. If the
gas leaves the manifold at a temperature of 450ยฐC and the muffler is located 2.6 m away, estimate the
temperature of the gas as it enters the muffler. Assume an ambient air temperature of 40ยฐC and a
heat transfer coefficient of 12 W/m2K for the outside surface of the exhaust pipe. The exhaust pipe is
thin walled and has a diameter of 10 cm. The exhaust pipe inner roughness Is ฮต = 0.020 mm.
Find: The approximate temperature of the exhaust gas as it enters the muffler (๐ป๐,๐).
Schematic:
Assumptions:
1. Incompressible, fully developed flow.
2. Constant properties of air at 723 K are assumed for the exhaust gas.
๐ = 0.4821 ๐๐/๐3 ๐๐ = 1.081 ร 10โ3๐ฝ/๐๐๐พ ๐๐ = 0.698
๐ = 3.461 ร 10โ5๐ โ ๐ /๐2 ๐ = 7.193 ร 10โ5๐2/๐ ๐ = 5.36 ร
10โ2 ๐/๐๐พ
Analysis:
๐ ๐๐ท =๐ข๐๐ท
๐=
6.0๐๐
ร 0.1 ๐
7.193 ร 10โ5๐2/๐ = 8.341 ร 103 > 2300
The flow is turbulent, but possibly transitional (not fully turbulent) since 2300 < ๐ ๐๐ท < 10,000.
๐
๐ท=
0.020 ร 10โ3๐
0.1๐= 0.0002
From Figure 8.3, the friction factor is:
๐ = 0.033
ME331: Introduction to Heat Transfer Spring 2017
For fully developed turbulent flow in a smooth tube, the local Nusselt number and local heat transfer
coefficient are:
๐๐ข๐ท =(
๐8
) (๐ ๐๐ท โ 1000)๐๐
1 + 12.7 (๐8
)
12
(๐๐2/3 โ 1)
= 25.586
โ๐ = ๐๐ข๐ท โ๐
๐ท= 13.714
๐
๐2 โ ๐พ
However, since ๐ ๐๐ท < 10,000 and since ๐ฟ
๐ท= 26 < 60, the calculated โ๐ could be either an overestimate
or an underestimate of โ๏ฟฝฬ ๏ฟฝ. Lacking any better estimate, use ๐๐ขฬ ฬ ฬ ฬ ๐ท โ ๐๐ข๐ท, โ๏ฟฝฬ ๏ฟฝ โ โ๐.
๐ ๐ก๐๐ก = ๐ ๐๐๐๐ฃ,๐ + ๐ ๐๐๐๐ฃ,๐ =1
โ๐๐๐ฟ+
1
โ๐๐๐ฟ=
1
๐ ร 0.1๐ ร 2.6๐(
1
13.714+
1
12) ๐2 โ
๐พ
๐= 0.19129
๐พ
๐
Then, from equation 8.45b,
๐โ โ ๐๐,๐
๐โ โ ๐๐,๐= exp (โ
1
๏ฟฝฬ๏ฟฝ๐๐๐ ๐ก๐๐ก)
๐๐,๐ = ๐โ โ (๐โ โ ๐๐,๐) ๐๐ฅ๐ (โ1
๏ฟฝฬ๏ฟฝ๐๐๐ ๐ก๐๐ก) = 371.4ยฐ๐ถ
๐๐,๐ = 370ยฐ๐ถ ANSWER
Comment:
๐ =ฮ๐๐๐
๐ ๐ก๐๐ก= โ1900 ๐
The negative sign indicates that heat flows from gas in the exhaust pipe to the ambient air environment.
ME331: Introduction to Heat Transfer Spring 2017
Problem 4
Statement: A blood purification system removes blood from a patient through a 3.0 mm diameter
tube at a mass flow rate of 0.090 kg/min. The blood exits the patient at 37ยฐC and cools to 33ยฐC just
prior to entering a filter and heat exchanger that raises the blood temperature back to 37ยฐC.
Approximate the properties of blood with those of water.
Find: Estimate the rate of heat transfer from the blood and the heat transfer coefficient, assuming
fully developed blood flow in the tube.
Schematic:
Assumptions:
1. Incompressible, fully developed flow.
2. Constant properties of water at 35ยฐC = 308.15 K are assumed for blood.
๐ = 993.8 ๐๐/๐3
๐ = 0.625 ๐/๐๐พ
๐ = 7.22 ร 10โ4 ๐ ๐ / ๐2
๐ = 4178 ๐ฝ/๐๐๐พ
3. Constant surface heat flux or constant surface temperature may be assumed.
Analysis:
๐ = ๏ฟฝฬ๏ฟฝ ๐ (๐๐,๐ โ ๐๐,๐) =0.090
60
๐๐
๐ ร 4178
๐ฝ
๐๐๐พร (33 โ 37)๐พ = โ25.07 ๐
๐ = โ25๐ ANSWER
The negative sign indicates that the blood is transfering heat to its surrounding environment.
The Reynolds number is:
๐ ๐๐ท =๐ข๐๐ท
๐=
๏ฟฝฬ๏ฟฝ๐ท
๐๐ด๐๐= 881.7 < ๐ ๐๐๐
ME331: Introduction to Heat Transfer Spring 2017
For fully developed laminar flow, constant surface heat flux, ๐๐ข๐ท =โ๐ท
๐= 4.36
โ = 4.36 โ๐
๐ท= 908.3
๐
๐2 โ ๐พ
โ = 910 ๐
๐2โ๐พ ANSWER
For fully developed laminar flow, constant surface temperature, ๐๐ข๐ท =โ๐ท
๐= 3.66
โ = 3.66 โ๐
๐ท= 762.5
๐
๐2 โ ๐พ
โ = 760 ๐
๐2โ๐พ ANSWER
Comments:
This process of blood cooling is unlikely to be either constant heat flux or surface temperature. It is also
unlikely for it to be fully developed. Therefore, these answers are first order estimates. If more
information is given about the outer convection condition, this problem could be solved more accurately
using a similar method as in problem 3.
ME331: Introduction to Heat Transfer Spring 2017
Problem 5
Statement: The condenser of a steam power plant contains N = 1000 brass tubes (kt = 110 W/m-K),
each of inner and outer diameter, Di = 25 mm and Do = 28 mm. Steam condensation on the outer
surfaces of the tubes is characterized by a convection coefficient of ho = 10,000 W/m2-K.
Find: (a) If cooling water from a nearby lake is pumped through the condenser tubes at a mass flow
rate of แนc = 400 kg/s, what is the overall heat transfer coefficient Uo based on the tube outer
surface area? Water properties may be approximated as constant: ฮผ = 9.60 x 10-4 N-s/m2, k =
0.60 W/m-K, and Pr = 6.6.
(b) If after extended operation, fouling results in an additional resistance of Rโf,I = 10-4 m2-K/W,
at the inner surface of the tubes, what is the value of Uo?
(c) If water is extracted from the lake at 15ยฐC, and 10 kg/s of steam at 0.0622 bar are to be
condensed, what is the temperature of the cooling (lake) water leaving the condenser? The
specific heat of the water is 4180 J/kg-K.
Schematic:
Assumptions:
1. Water is incompressible, and viscous dissipation is negligible.
2. Fully developed flow in tubes.
3. Negligible fouling on outer tube surfaces, Do.
4. Water properties inside tubes are given as follows:
c = 4180 J/kgโ K, ฮผ = 9.6 ร 10-4 Nโ s/m2, k = 0.60 W/mโ K, Pr = 6.6.
5. From Table A-6, properties outside tubes are for saturated vapor / liquid water at p = 0.0622
bar: Tsat = 310 K, hfg = 2.414 ร 106 J/kg.
ME331: Introduction to Heat Transfer Spring 2017
Analysis:
(a) Without fouling resistance, Eq. 11.5 yields:
1
๐๐=
1
โ๐(
๐ท๐
๐ท๐) +
๐ท๐ ln (๐ท๐๐ท๐
)
2๐๐ก+
1
โ๐
๐ ๐๐ท๐=
4๏ฟฝฬ๏ฟฝ1
๐๐ท๐๐= 21,220; thus the flow in the tubes is turbulent. From eq 8.60:
โ๐ = (๐
๐ท๐) 0.023๐ ๐๐ท๐
4/5๐๐0.4 = 3398 ๐/๐2๐พ
๐๐ = [1
โ๐(
๐ท๐
๐ท๐) +
๐ท๐ ๐๐(๐ท๐๐ท๐
)
2๐๐ก+
1
โ๐]
โ1
= [1
3398 ๐/๐2๐พ(
28
25) +
0.028๐ ๐๐(28
25)
2ร110 ๐/๐๐พ+
1
10,000 ๐/๐2๐พ]
โ1
= 2252๐
๐2๐พ
๐๐ = 2250๐
๐2๐พ ANSWER
(b) With fouling, Eq. 11.5 yields:
1
๐๐=
1
โ๐(
๐ท๐
๐ท๐) +
๐ท๐ ln (๐ท๐๐ท๐
)
2๐๐ก+
1
โ๐+ (
๐ท๐
๐ท๐) ๐ ๐,๐
โฒโฒ
๐๐ = [4.44 ร 10โ4๐2๐พ/๐ + (๐ท๐
๐ท๐) ๐ ๐,๐
โฒโฒ ]โ1
= 1798 ๐
๐2๐พ
๐๐ = 1800 ๐
๐2๐พ ANSWER
(c) The rate of heat transfer to water inside the tubes equals the rate at which thermal energy is
extracted from steam outside of the tubes.
๏ฟฝฬ๏ฟฝโโ๐๐ = ๏ฟฝฬ๏ฟฝ๐ ๐ (๐๐,๐ โ ๐๐,๐)
๐๐,๐ = ๐๐,๐ +๏ฟฝฬ๏ฟฝโโ๐๐
๏ฟฝฬ๏ฟฝ๐๐= 15ยฐ๐ถ +
10๐๐
๐ ร2.414ร106 ๐ฝ
๐๐
400๐๐
๐ ร4180
๐ฝ
๐๐๐พ
= 29.4ยฐ๐ถ
๐๐,๐ = 30ยฐ๐ถ ANSWER
Comments: (1) The largest contribution to the thermal resistance is from convection at the interior
surface of the tubes. To increase Uo, hi could be increased by increasing ๏ฟฝฬ๏ฟฝ๐. (2) Note that the outlet
temperature of the cooling water Tm,o = 29.4ยฐ๐ถ = 302.5 K remains below Tsat = 310 K, as required.
ME331: Introduction to Heat Transfer Spring 2017
Problem 6
Statement: In a Rankine power system, 1.5 kg/s of steam leaves the turbine as saturated vapor at 0.51
bar. The steam is condensed to saturated liquid by passing it over the tubes of a shell-and-tube heat
exchanger, while cooling liquid water, having an inlet temperature of Tc,I = 280 K, is passed through
the tubes. The condensing heat exchanger contains 100 thin-walled tubes, each 10 mm diameter, and
the total cooling water flow rate through the tubes is 15 kg/s. The average convection coefficient
associated with condensation on the outer surface of the tubes may be approximated as โo = 5000
W/m2-K. Appropriate property values for the cooling liquid water are c = 4178 J/kg-K, ฮผ = 700 x 10-6
kg/s-m, k = 0.628 W/m-K, and Pr = 4.6.
Find: (a) What is the liquid water outlet temperature from the tubes?
(b) What is the required tube length (per tube)?
(c) After extended use, deposits accumulating on the inner and outer tube surfaces result in a
cumulative fouling factor of 0.0003 m2-K/W. For the prescribed inlet conditions and the
computed tube length, what mass fraction of the vapor is condensed?
Schematic:
Assumptions:
1. Negligible heat loss to the surroundings.
2. Negligible wall conduction resistance and fouling (initially). With fouling, Rfโโ = 0.0003 m2โ K/W.
3. Cooling liquid water properties treated as constant:
c = 4178 J/kgโ K, ฮผ = 700 ร 10-6 kg/sโ m, k = 0.628 W/mโ K, Pr = 4.6.
4. From Table A.6, for saturated steam at p = 0.51 bar, Tsat = 355 K, hfg = 2.304 ร 106 J/kg.
Analysis:
(a) From an overall energy balance, ๐โ = ๏ฟฝฬ๏ฟฝโโ๐๐ = ๐๐ = ๏ฟฝฬ๏ฟฝ๐๐๐(๐๐,๐ โ ๐๐,๐)
ME331: Introduction to Heat Transfer Spring 2017
๐๐,๐ = ๐๐,๐ +๏ฟฝฬ๏ฟฝโโ๐๐
๏ฟฝฬ๏ฟฝ๐๐๐= 280 ๐พ +
1.5๐๐๐
ร 2.304 ร 106 ๐ฝ๐๐
15๐๐๐
ร 4178๐ฝ
๐๐
= 335.1 ๐พ
๐๐,๐ = 335 ๐พ ANSWER
(b) With ๐ถ๐ = ๐ถ๐๐๐/๐ถ๐๐๐ฅ โ 0, ๐๐๐ = โln (1 โ ๐),
where ๐ =๐
๐๐๐ด๐=
๏ฟฝฬ๏ฟฝ๐๐๐(๐๐,๐ โ ๐๐,๐)
๏ฟฝฬ๏ฟฝ๐๐๐(๐โ,๐ โ ๐๐,๐)=
335.1 โ 280
355 โ 280= 0.7347
๐๐๐ = โ ln(1 โ 0.7347) = 1.327 = ๐๐ด/๐ถ๐๐๐
With negligible tube wall thickness, the overall heat transfer coefficient U is: 1
๐=
1
โฬ ๐+
1
โฬ ๐.
The Reynolds number for internal flow inside a singe tube is:
๐ ๐๐ท =4๏ฟฝฬ๏ฟฝ๐
๐๐ท๐=
4 ร 15๐๐๐
/100
๐(0.01) ร 700 ร 10โ6๐๐/๐ โ ๐= 27,284
Using the Dittus-Boelter correlation (equation 8.60) for fully developed turbulent pipe flow,
๐๐ข๐ท = 0.023๐ ๐๐ท4/5
๐๐๐ = 0.023(27,284)4/5(4.6)0.4 = 149.8
Assuming the tube length is sufficiently long such that the internal flow is fully developed for the
majority of the tube length (L > 60 D):
โ๏ฟฝฬ ๏ฟฝ โ โ๐ = (๐
๐ท) ๐๐ข๐ท = 9,408
๐
๐2๐พ
With ๐ = [(1
9408) + (
1
5000)]
โ1 ๐
๐2๐พ= 3,265
๐
๐2๐พ, the heat transfer area is:
๐ด = ๏ฟฝฬ๏ฟฝ๐๐๐ (๐๐๐
๐) = 15
๐๐
๐ (4178
๐ฝ
๐๐๐พ) (
1.327
3,265๐
๐2๐พ
) = 25.5๐2
The tube length is ๐ฟ =๐ด
๐๐๐ท=
25.5๐2
100๐(0.01๐)= 8.11 ๐, satisfying the condition that L > 60 *0.01 m = 0.6 m.
๐ฟ = 8.1 ๐ ANSWER
(c) With fouling, the overall heat transfer coefficient is 1/๐๐ค๐ = 1/๐๐ค๐๐ + ๐ ๐โฒโฒ.
Hence,
1
๐๐ค๐= (3.063 ร 10โ4 + 3 ร 10โ4)๐2 โ ๐พ/๐
ME331: Introduction to Heat Transfer Spring 2017
๐๐ค๐ = 1649 ๐/๐2 โ ๐พ
๐๐๐ = ๐๐ค๐๐ด/๐ถ๐๐๐ = (1649 ๐/๐2 โ ๐พ ร 25.5๐2)/( 15 ๐๐ /๐ ร 4178 ๐ฝ /๐๐ โ ๐พ) = 0.670
From equation 11.35a,
๐ = 1 โ exp(โ๐๐๐) = 1 โ exp(โ0.670) = 0.488
๐ = ๐๐๐๐๐ฅ = 0.488 ร 15๐๐
๐ ร 4178
๐ฝ
๐๐โ๐พ(355 โ 280)๐พ = 2.296 ร 106๐
Without fouling, the heat rate was:
๐ = ๏ฟฝฬ๏ฟฝโโ๐๐ = 1.5๐๐
๐ ร 2.304 ร 106
๐ฝ
๐๐= 3.456 ร 106๐
Hence,
๏ฟฝฬ๏ฟฝโ,๐ค๐
๏ฟฝฬ๏ฟฝโ,๐ค๐๐=
2.296 ร 106
3.456 ร 106= 0.664
The reduced condensation rate with fouling is then:
๏ฟฝฬ๏ฟฝโ,๐ค๐ = 0.664 ร 1.5๐๐
๐ = 0.996
๐๐
๐
๏ฟฝฬ๏ฟฝโ,๐ค๐ = 1.0๐๐
๐ ANSWER