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Assignment for practical 1
Air recirculation through a duct-room system
Table of Contents
1. Assignment: Air recirculation through a duct-room system ....................................................................... 1
Before class ...................................................................................................................................................... 1 Note: ................................................................................................................................................................ 1
Assignment details .......................................................................................................................................... 2
2. Instruction of Practical tests ........................................................................................................................ 2 Abstract ........................................................................................................................................................... 2 Equipment ....................................................................................................................................................... 3 Assumptions .................................................................................................................................................... 3
Test A ............................................................................................................................................................... 4 Test B ............................................................................................................................................................... 4 Appendix A .......................................................................................................................................................... 5 Appendix B ........................................................................................................................................................... 5
1. Assignment: Air recirculation through a duct-room system
Before class
Please note that attending practical class is compulsory. Not attending the Practical class will lead to a zero mark. Before attending the practical class, students need to
(1) Whatch the recorded practical videos available on vUWS;
(2) Download and read this lab instruction document from vUWS;
(3) Download the excel file for generating experimental data.
In the Practical class, complete Test A and Test B.
Note:
• You must use your own data to do analysis. The marks will be significantly reduced if there are mistakes in the recorded data, or zero if all the data are wrong. Copying other data deliberately or accidently will lead to a zero mark.
• You need to use the theory about thermal and fluid engineering, thermodynamics and fluid mechanics to complete the assignment and write the report.
• You need to explain how the results are calculated, in addition to giving formulae and results. This report tests students’ ability to solve simple problem of heat transfer.
• To explain the results and present the data clearly and informatively, you need to use diagrams, curves or tables.
Assignment details
Conduct Test A and B following the instruction and perform the following analysis and write a minimum 2500-word comprehensive report. In the following questions, heat loss means heat loss per unit time (in Watts or W).
(1) In the report you need to explain how the values, including the heat flow rates and heat losses, are calculated. The formulae and theory must be given. Only showing the results is not satisfactory. (20% marks)
(2) In test A (35% marks) o Calculate the heat loss between T2 and T4 and the heat loss between T4 and T5 (see
Figure 1 for the positions of T1 – T5) for all the tests; o Calculate the heat supplied by the preheater; o Discuss how these heat losses are affected by temperature T2; o Compare the total heat loss with the heat supplied.
o Analysis the results and discuss how the preheater power affect the temperatures and heat losses.
o When the preheater power is constant and fan speed varies, discuss the effect of the mass flow rate on the temperatures and heat loss.
(3) In test B (35% marks) o For all the tests, calculate the heat supplied by the preheater, the heat loss between T5 and T1, heat loss between T4 and T5 and the total heat loss through the whole route T2-
T3-T4-T5-T1; o Compare the total heat loss with the heat supplied by the preheater; o Discuss how these heat losses are affected by the temperature T2.
o Dicsucs how the temperature and heat are affected by the mass flow rate and preheater power.
(4) Compare the temperatures and heat losses in Test A with those in Test B and analyze the reason for the difference. (10% marks)
Appendix A lists some important formulae (not all) for air flow in a duct.
Appendix B is a working table that can be used to record the experimental data.
2. Instruction of Practical tests
Abstract
In this Practical class, the air flow through a duct and a room will be studied. The heat loss through a duct system will be analyzed. Two sets of tests will be conducted. In test A, heated air flows into a room through a duct and flows out through an exit. In test B, air flows through a recirculated ductroom system.
(a) Side view
(b) Top view
Figure 1 Sketch of the Air conditioning unit
Equipment
Figure 1 shows a sketch of the Air conditioning unit. It has a recirculation duct, a room model, two heaters (preheater and reheater), two velocity meters (Velocity u1 and velocity u2) and five temperature sensors (T1 to T5). The sensors RH1 to RH5 are used to measure Relative Humidity. They will not be used in this study. Three louvres (inlet, outlet and recycle) are used to control the flow.
The cross section area of the duct is 0.2 m×0.2 m = 0.04 m2.
The air conditioning unit is controlled by computer software. The power of the heaters, the speed of the fans can be controlled using a computer. The measured temperature and flow velocity can be read directly from the software as shown in Figure 1 (b).
Assumptions
The following assumptions are used in this study.
(1) The air is dry air.
(2) The flow is uniform in the duct and the temperature is uniformly distributed in the cross sections of the duct.
P
(3) The air density can be calculated based on the ideal gas law: ?RT , where the pressure is
? the atmospheric pressure (Patm=101325 Pa) and R is the specific gas constant of air (R=287 J/kg?K).
(4) The specific heat of the air varies with the temperature and it can be calculated by
28.11? 0.1967 ?10?2T ? 0.4802?10?5T2 ? 0.1966?10?9T3
cp=1000cp ?×
28.97
where the temperature T is in Kelvin (K) and cp is in J/kg?K.
Note: In the following steps, the number n stands for the last digit of your student ID. For example, if
your student ID is 1233243, (50+2n)%= (50+2?3)%=56% and 50% if your student ID is 123450.
Test A
In test A, air enters the duct through inlet louvre and exit through the outlet louvre. The system is not a recirculated system. Before the air enters the room, it is heated by the preheater.
• Inlet Louver fully open; outlet louvre fully open, recycle louvre fully close.
• Set the fan speed to (50+n)%, and set the preheater control to (20+2n)%. Wait until the temperatures are stabilized (you do not need to wait if you use excel file to generate the data). Record the temperatures and the flow velocities. Keep the fan speed unchanged, repeat the tests for (30+2n)%, (40+2n)%, (50+2n)%, (60+2n)%, (70+2n)% and (80+2n)%.
• Set the preheater control to (40+2n)% and keep it constant, change the fan speed to from (30+n)% to (90+n)% with an increament of 10%. For each fan speed, record the temperatures and flow velocities.
Note: The temperatures will be stabilized very slowly in thphysical tests. You must ensure they are stabilized before you record the data. If you use the excel file to generate the data, the temperature are stabilized temperature immediately after you set up the input data.
Test B
In test B, the air flows through a recirculation duct.
(1) Close the Inlet Louvre and outlet Louvre, open Recycle Louvre fully. The system becomes an air recirculation system. Set the fan speed to (50+n)% amd keep it constant. Set the preheater control to (20+n)% to (90+n)%, with an interval of 10%. For each preheater power, record the temperatures and flow velocities.
(2) The system is still a air recirculation system. Set the preheater control to (40+2n)% and keep it constant. Change the fan speed to from (30+n)% to (90+n)% with an increament of 10%. For each fan speed, record the temperatures and flow velocities.
Appendix A
The difference in energy (heat) between positions A and B in a duct can be calculated by (for dry air and without mass loss between A and B)
?Q?AB ?m??cpBTB ?cpATA?
where cp is the specific heat for constant pressure and T is the temperature, m??is the mass flow rate. The mass flow rate can be calculated by
m????VA
where ? is the density of the fluid, V is the flow velocity and A is the cross sectional area.
Appendix B
You may use the following working table to record the data (you may need to increase the row number for large number of tests).
Test Speed of the fan (%) relative to full speed Power of
the preheater (%) relative to the full power V1
(m/s) V2
(m/s) T1
(°C) T2
(°C) T3
(°C) T4
(°C) T5
(°C)