Program: MSc Mechanical Engineering, Meng Mechanical Engineering Module
Title & Code: Advanced Thermo-fluid Application-GEEN1121
Level: 7
Date: April/May 2021
University of Greenwich
Questio n | Solution | Mark |
Q1 | Solution (a) ππ = is the log mean temperature difference and it is expressed as Ξπ1β Ξπ2 ππ = ππ(Ξπ βΞπ ) 1 2 Where, Ξπ1 = πβ,ππ β ππ,ππ Ξπ2 = πβ,ππ’π‘ β ππ,ππ’π‘ for parallel flow Heat exchanger Ξπ1 = πβ,ππ β ππ,ππ’π‘ Ξπ2 = πβ,ππ’π‘ β ππ,ππ for counter flow Heat exchanger Solution (b) The heat exchangers usually operate for long periods of time with no change in their operating conditions, and then they can be modeled as steady-flow devices. As such, the mass flow rate of each fluid remains constant and the fluid properties such as temperature and velocity at any inlet and outlet remain constant. The kinetic and potential energy changes are negligible. The specific heat of a fluid can be treated as constant in a specified temperature range. Axial heat conduction along the tube is negligible. Finally, the outer surface of the heat exchanger is assumed to be perfectly insulated so that there is no heat loss to the surrounding medium and any heat transfer thus occurs is between the two fluids only. Solution (c) Heat transfer from exhaust gas can be determined by (i) π = πΜ πππ Γ ππ,πππ Γ (ππππ ,ππ β ππππ ,ππ’π‘) 200ππ β = ( ) ( ) β 3600 π Γ (1.13 ππ½βππ β πΎ) Γ (350 β 100)πΎ = 15.694 ππ Let the exit temperature of the water to be t exit, therefore, | 5 5 3 |
1400ππ β 15.694 ππ = ( )( ) Γ (4.19 ππ½βππ β πΎ) Γ (π‘ππ₯ππ‘ β 10)πΎ β 3600 π Rearranging we have 15.694 ππ (0.389 Γ 4. ) = (π‘ππ₯ππ‘ β 10)πΎ 19 π‘ππ₯ππ‘ = 9.63 + 10 = 19.63 ππΆ The overall heat transfer coefficient is determined from the following equation 1 1 1 1 = + = 4.0 π2 πΎβππ πππ π = = 0.25 ππβπ2 πΎ π 0.3 1.5 4 The logarithmic mean effective temperature can be found (for parallel flow) by Ξπ1β Ξπ2 ππ = ππ(Ξπ βΞπ ) 1 2 Ξπ1 = πβ,ππ β ππ,ππ Ξπ2 = πβ,ππ’π‘ β ππ,ππ’π‘ for parallel flow Heat exchanger Ξπ1 = πβ,ππ β ππ,ππ = 350 β 10 = 340ππΆ Ξπ2 = πβ,ππ’π‘ β ππ,ππ’π‘ = 100 β 19.63 = 80.37ππΆ 340 β 80.37 ππ = = 180.1ππΆ ππ(340β80.37) The heat transfer can also be determined from π = ππ΄ππ π΄ = πππΏ Rearranging the above equation in terms of L we have π 15.694 πΏ = = = 1.48 π πππππ π Γ 0.075 Γ 0.25 Γ 180.1 The logarithmic mean effective temperature can be found (for counter flow) by | 3 5 |
Ξπ1β Ξπ2 ππ = ππ(Ξπ βΞπ ) 1 2 Ξπ1 = πβ,ππ β ππ,ππ’π‘ Ξπ2 = πβ,ππ’π‘ β ππ,ππ for counter flow Heat exchanger Ξπ1 = πβ,ππ β ππ,ππ’π‘ = 350 β 19.63 = 330.37ππΆ Ξπ2 = πβ,ππ’π‘ β ππ,ππ = 100 β 10 = 90ππΆ 330.37 β 90 ππ = = 184.84ππΆ ππ(330.37β90) The heat transfer can also be determined from π = ππ΄ππ π΄ = πππΏ Rearranging the above equation in terms of L we have π 15.694 πΏ = = = 1.44π πππππ π Γ 0.075 Γ 0.25 Γ 184.84 | 4 | |
Q2 | Solution (a) The vorticity is twice the average angular rotation of a fluid element. It can be calculated by taking the Curl of the velocity vector. Solution (b-i) π = ππΏπ(π) ππ π π£π = ππ = π 1 ππ π£π = π ππ = 0 Solution (b-ii) π 1 ππ ππ π£π = π = π ππ βΉ ππ = π | 4 marks 4 marks |
Integrate with respect to π: π = ππ + π1(π) On the other hand, π£ = 0 = β ππ π ππ Therefore, π is not function of π, so π1(π) is a constant and the equation for π becomes: π = ππ + πΆ1 πΆ1 is an arbitrary constant. Also, π = 0 for π = π 3 Thus, 0 = π Γ π + πΆ βΉ πΆ = βπ Γ π 3 1 1 3 π π π = ππ β π = π(π β ) 3 3 Solution (b-iii) At point π΅, π = βπ₯2 + π¦2 = β12 + 42 = 4.12 π = tanβ1 π¦) = tanβ1(4) = 1.33 ( π₯ The value of the stream function at B: π ππ΅ = β0.71 = π(1.33 β 3) π = β2.51 | 12 marks 5 marks | |
Q3 | Solution (a) (i) | 6 |
T (oC) | s (kJ/kg-K) | h (kJ/kg) |
10 | 0.9305 ππ½βππ β πΎ | 256.58 |
12.23 | 0.93773 ππ½βππ β πΎ | 258.69 |
20 | 0.9628 ππ½βππ β πΎ | 265.86 |
(ii) Solution Q3 (a-ii) Determine the properties at different states. State1: Refer the following value for saturated vapour of Refrigerant 134a at π1 = β10.09ππΆ from property table, we can read. β1 = βπ = 244.46 ππ½βππ π 1 = π π = 0.93773 ππ½βππ β πΎ State2: the compression process through compressor 1 is isentropic that is π 2 = π 1 = 0.93773 ππ½βππ β πΎ From superheated table at 5 bar (0.4 MPa) we read By interpolation assuming linearity find the temperature and enthalpy 20 β π 0.9628 β 0.93773 = 20 β 10 0.9628 β 0.9305 20 β π = 0.777 10 π2 = 20 β (10 Γ 0.77) = 12.23ππΆ For enthalpy 265.86 β β 0.9628 β 0.93773 = 265.86 β 256.59 0.9628 β 0.9305 |
265.86 β β 9.27
= 0.777
β2 = 265.86 β (9.27 Γ 0.77) = 258.69 ππ½βππ
state5: refrigerant is leaving the condensed as saturated liquid at π5 = 8 πππ
β5 = βπ = 95.47 ππ½βππ
State6: throttling of the refrigerant and enthalpy remains constant and therefore,
β6 = 95.47 ππ½βππ
We can determine the quality of the steam by reading the enthalpy value from the property table at π6 = 4 πππ pressure.
βπ = 63.94 ππ½βππ βππ = 191.62 ππ½βππ
(95.47 β 63.94) ππ½βππ
π₯6 =
191.62 ππ½βππ
= 0.165
State7: Refer the following value for saturated liquid at π7 = 4 πππ
from property table:
β7 = βπ = 63.94 ππ½βππ
State8: Throttling occurs as the refrigerant flows through the expansion valve.
Therefore,
β8 = β7 = 63.94 ππ½βππ
State3: Refer the following value for saturated vapour at π3 = 4 πππ
from property table,
β3 = βπ = 259.55 ππ½βππ
State 9: The fraction of the flow into the flash chamber at state 6 that exits as saturated vapour at state 3 is equal to the quality at state 6 (π₯6). The liquid leaving the flash chamber at state 7 is the fraction (1 β π₯6). With these flow rates ratio,
0 = (1 β π₯6)β2 + π₯6β3 β 1β9 β9 = (1 β π₯6)β2 + π₯6β3
β9 = (1 β 0.165)(258.69 ππ½βππ) + (0.165)(259.55 ππ½βππ) = 258.83 ππ½βππ
Use the table to obtain the value of specific entropy corresponding to π9 = 4 πππ and β9 = 258.83 ππ½βππ βΆ
From superheated table at 8 bar (0.8 MPa) we read
T (oC) | s (kJ/kg-K) | h (kJ/kg) |
10 | 0.9305 ππ½βππ β πΎ | 256.58 |
12.42 | 258.83 |
20 | 0.9628 ππ½βππ β πΎ | 265.86 | 8 6 | |||
By interpolation assuming linearity find the temperature and enthalpy 20 β π 265.86 β 258.83 = 20 β 10 265.86 β 256.58 20 β π = 0.758 10 π2 = 20 β (10 Γ 0.758) = 12.42ππΆ For enthalpy 0.9628 β π 0.9628 β 0.93773 = 0.9628 β 0.9305 0.9628 β 0.9305 0.9628 β π = 0.758 0.0323 π 9 = 0.9628 β (0.0323 Γ 0.758) = 0.9383 ππ½βππ β πΎ State4: The compression process through compressor 2 is isentropic that is π 3 = π 4 = 0.9383 ππ½βππ β πΎ. β4 = 273.5 ππ½βππ βΆ From superheated table at 8 bar (0.8 MPa) we read T (oC) s (kJ/kg-K) h (kJ/kg) 31.31 0.9183 267.29 37.2 0.9383 273.5 40 0.9480 276.45 By interpolation assuming linearity find the temperature and enthalpy 40 β π 0.9480 β 0.9383 = 40 β 31.31 0.9480 β 0.9183 40 β π = 0.327 8.69 π4 = 40 β (8.69 Γ 0.327) = 37.2ππΆ For enthalpy 276.45 β β 0.9480 β 0.9383 = 276.45 β 267.29 0.9480 β 0.9183 276.45 β β = 0.327 9.16 β4 = 276.45 β (9.16 Γ 0.327) = 273.5 ππ½βππ (iii) |
Initially determine the mass flow rates to determine the compressor power. For the evaporator, πΜππ = πΜ 1(β1 β β8) πΜππ 10 π‘πππ 211 ππ½βπππ 1 πππ πΜ 1 = (β β β ) = (244.46 β 63.94) ππ½βππ [ 1 π‘ππ ] ( 60 π ) 1 8 = 0.195 ππβπ Also, since (1 β π₯6) is the fraction of the total flow passing through the evaporator, πΜ1 = (1 β π₯ ) πΜ 3 6 πΜ 1 0.195 ππβπ πΜ 3 = (1 β π₯ ) = (1 β 0.165) = 0.234 ππβπ 6 (iv) Power input to compressor 1 πΜ ππππ,1 = πΜ 1(β2 β β1) = (0.195 ππβπ )(258.69 β 244.46) ππ½βππ = 2.77 ππ Power input to compressor 2 πΜ ππππ,2 = πΜ 2(β4 β β3) = (0.234 ππβπ )(273.5 β 259.55) ππ½βππ = 3.26 ππ (v) The coefficient of performance is πΜππ πΆ. π. π = πΜ ππππ,1 + πΜ ππππ,2 10 π‘πππ 211 ππ½βπππ 1 πππ 1 ππ = (10.31 + 6.64) ππ½βππ [ 1 π‘ππ ] ( 60 π )(1 ππ½βπ ) = 5.8 | 3 | |
2 | ||
Q4 | Solution (a) | |
A heat exchanger is classified as being compact if ο’ > 700 m2/m3 or (200 ft2/ft3) where ο’ is the ratio of the heat transfer surface area to its volume which is called the area density. The area density for double-pipe heat exchanger can not be in the order of 700. | 2 | |
Solution (b) No, it cannot be classified as a compact heat exchanger. As the heat transfer area to volume ratio is below 700 m2/m3 | 2 |
Solution (c) | ||
In counter-flow heat exchangers, the hot and the cold fluids move | ||
parallel to each other, but both enter the heat exchanger at | 2 | |
opposite ends and flow in opposite direction. In cross-flow heat | ||
exchangers, the two fluids usually move perpendicular to each | ||
other. | ||
Solution (d) | ||
1 | ||
The cross-flow is said to be unmixed when the plate fins force the | ||
fluid to flow through a particular inter fin spacing and prevent it | ||
from moving in the transverse direction. When the fluid is free to | ||
move in the transverse direction, the cross-flow is said to be mixed. | ||
Solution (e) | ||
In the shell and tube exchangers, baffles are commonly placed in | 5 | |
the shell to force the shell side fluid to flow across the shell to | ||
enhance heat transfer and to maintain uniform spacing between | ||
the tubes. Baffles disrupt the flow of fluid, and an increased | ||
pumping power will be needed to maintain flow. On the other hand, | ||
baffles eliminate dead spots and increase heat transfer rate. | ||
Solution (f) | ||
Open question | ||
Q5 | Solution (a) Applying Bernoulliβs equation from far upstream to just upstream the rotor: 1 1 ππ + ππ2 = π + ππ’2 2 π 2 It is also valid to apply Bernoulliβs equation from just behind the rotor to the far downstream wake. 1 1 π β βπ + ππ’2 = ππ + ππ’2 2 2 1 |
Combining the two equations yields: π + 1 ππ2 + π β βπ + 1 ππ2 + 1 ππ’2 = π + π + 1 ππ’2 + 1 ππ’2 π 2 π 2 π 2 π 2 2 1 1 1 βΉ ππ2 β βπ = ππ’2 2 π 2 1 βΉ βπ = 1 π(π2 β π’2) 2 π 1 Therefore the thrust on the rotor is: 1 π = ππ΄(π2 β π’2) 2 π 1 Now applying the rate of change of momentum: π = πΜ (ππ β π’1) Since πΜ = ππππ΄π = ππ’π΄ = ππ’1π΄1 Implies π = ππ’π΄(ππ β π’1) Combining the two equations for the thrust yields: 1 ππ΄(π2 β π’2) = ππ’π΄(π β π’ ) 2 π 1 π 1 1 βΉ (ππ β π’1)(ππ + π’1) = π’(ππ β π’1) βΉ 2 1 π’ = (ππ + π’1) 2 Solution (b) ππ = 12ππ β1 π = 0.21 ππ β π’1 π = βΉ πππ = ππ β π’ ππ π’ = ππ β πππ = ππ(1 β π) π’ = 12(1 β 0.21) | 12 marks 4 marks |
π’ = 9.48ππ β1 Solution (c) 1 π’ = (ππ + π’1) βΉ 2π’ = ππ + π’1 2 π’1 = 2π’ β ππ π’1 = 2ππ(1 β π) β ππ = 2ππ β 2πππ β ππ π’1 = ππ β 2πππ βΉ π’1 = ππ(1 β 2π) π’1 = 12(1 β 2 Γ 0.21) βΉ π’1 = 6.96ππ β1 Solution (c) ππ·2 π = ππ’π΄(ππ β π1) βΉ π = ππ’ 4 (ππ β π’1) 1.2 Γ 9.48 Γ 3.14 Γ 62 Γ (12 β 6.96) π = βΉ 4 π = 1620.28π | 6 marks 3 marks | |