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EXAMINATION PAPER: ACADEMIC SESSION 2019/2020
Campus: Medway
Faculty: Engineering & Science
Level; 5
Exam Session: April/May 2020
MODULE CODE MECH1061
MODULE TITLE: THERMODYNAMICS
Type of Examination: Take Home Exam
SECTION A
Answer ALL questions in Section A
Q1.
A refrigerant (R-134a) enters a water-cooled condenser at a flow rate of 0.2 kg/s. The pressure of the refrigerant at the inlet of the condenser is 1 MPa and 60 oC. The temperature of the refrigerant at the exit of the condenser is 35oC. Cooling water enters the condenser at 10 oC and exits at 20 oC.
(a) State assumptions/ relevant equations and sketches when determining enthalpies of water and refrigerant. [5 marks]
(b) Draw condensation process in p-h diagram provided at the end of the question paper (Note: write your name/ student ID on p-h diagram and attach to exam). [5 marks]
(c) Determine enthalpies of water and refrigerant and calculate the water flow rate through the condenser. [5 marks]
(d) Provide a brief description of the thermodynamic concept of refrigeration supported by diagrams. [5 marks] [Total 20 marks]
ANSWER(Purchase full paper to get all the solution)
1a)
ASSUMPTIONS
Equations
H=E + PV (Where p is pressure & V volume)
Enthalpy= specific enthalpy × Mass
.
1b)
1c)
Using Steam Cable
Enthalpy of refrigerant at inlet temperature
Enthalpy of refrigerant at exit temperature
Enthalpy of Water at inlet temperature
Enthalpy of water at exit temperature
The rate at which refrigerant enters the condenser() = 0.2kg/s
The rate at which cooling water flows through the condenser =
=
0.2
0.2 4.5957
= 0.919kg/s
1d)
Thermodynamic concept of refrigeration
A refrigeration cycle is entirely based on thermodynamic: from the removal of heat from one body (object or substance), transferring it to another. An example of refrigeration cycle is the Carnot refrigeration cycle , which operate between two given temperature.
It receive energy at higher temperature convert a portion of the energy to work and reject the remainder to sink at lower temperature.
Carnot Refrigeration Cycle.
Q2.
A heat engine receives a heat transfer rate of 1 MW at a high temperature of 550 oC and rejects heat to the ambient surrounding at 300 K. 450 kW of power are produced.
(a) State assumptions/ relevant equations and sketches when calculating the energy rejected to the surrounding. [4 marks]
(b) Calculate the energy rejected to the ambient surrounding. [5marks]
(c) Determine the Carnot efficiency and the thermal efficiency. [5 marks]
(d) Explain the concept of heat engines and irreversible processes and discuss the difference between the thermal and Carnot efficiency. [6 marks] [Total 20 marks]
Q3.
Consider a simple steam power plant. The following data (Table Q4) are:
Table Q3 – Process Data Simple Steam Power Plant
(a) State assumptions/ relevant equations and sketches when determining when determining the missing values for Table Q3 (a-f). [3 marks]
(b) Determine all missing values for Table Q3 (a-f). [6 marks]
(c) Calculate the heat transfer in the condenser and the boiler. [4 marks]
(d) Determine the ideal and real turbine work when a turbine efficiency of 80 percent can be assumed. [4 marks]
(e) Explain the difference between an open and a closed system and discuss which applies to the Rankine cycle and why. [3 marks] [Total 20 marks]
Q4.
Steam enters a steam turbine at a pressure of 1 MPa and a temperature of 300 oC. The velocity of steam at the turbine inlet is 50 m/s. At the exit of the turbine steam has a pressure of 150 kPa and a velocity of 200 m/s.
(a) State assumptions/ relevant equations and sketches when determining enthalpies of steam at the inlet and exit of the steam turbine. [4 marks]
(b) Draw the steam expansion process in a T-s diagram. [4 marks]
(c) Determine enthalpies of the steam at the inlet and exit. [4 marks]
(d) Calculate the work per kg of steam flowing through the turbine. [4 marks]
(e) Provide a short description and diagrams explaining the difference between an ideal and a real (non- adiabatic) turbine. [4 marks] [Total 20 marks]
Q5.
Explain the working principle of a gas turbine cycle using a P-v diagram and discuss the four stages in the gas turbine cycle.
Q6.
Draw the ideal Rankine cycle in a T-s diagram and explain the four stages of the cycle. Show that an increase in the boiler temperature will increase the total cycle efficiency. [4 marks]
Q7.
Draw an isotherm in a P-v diagram and explain the concept of reversibility of a thermodynamic process.
Q8.
Write down the ideal gas law and list the assumptions made when using the ideal gas law for calculations.
Q9.
Write down the non-flow energy equation (closed system) and discuss the difference to the steady flow energy equation (open system).
Q10.
Sketch a counter current flow heat exchanger and explain why this configuration is more efficient than a parallel flow heat exchanger.
Q11.
Draw a P-v diagram of an ideal Otto cycle and explain the four stages of the process.
Q12.
Write down the first law of thermodynamics and show which terms can be cancelled out from this equation when heat is added to a closed tank containing nitrogen.
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Last updated: Jun 23, 2021 10:49 AM
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