1.0 Lab 1: Heating Value Measurements: Solid and Liquid Fuels with a Bomb Calorimeter
1.1 Introduction
Measurements of heating values can be derived for both solid and liquid combustible fuels using an Oxygen Bomb Calorimeter. The calorimeter measures heat. Heat is not the same thing as temperature. The heat of combustion is called the calorific value. The calorific value is “:the number of heat units liberated by a unit mass of a sample when burned with oxygen in an enclosure of constant volume” (Introduction 2007). The experiment uses the 6100 Bomb Calorimeter. The definition of calorific value in a bomb calorimeter is “the heat liberated by the combustion of all carbon and hydrogen with oxygen to form carbon dioxide and water, including the heat liberated by other elements such as sulfur which may be present in the sample” (Introduction 2007). At the beginning of the combustion experiment the oxygen and the sample measure the same temperature, at the end the temperature the sample and oxygen is reduced to the initial temperature. Water vapor formed during the process is condensed to a liquid. The units for heat energy are reported as Joules (J). The bomb calorimeter is composed of four main parts. The first is the ‘bomb’’ which is the vessel where the samples are combusted. (See fig. A-1) The second is the ‘bucket’ the container that holds the bomb in a known quantity of water. The third is the insulating jacket that ensures a consistent temperature within the bucket. The fourth is a temperature sensor or a thermometer that can reliably measure the temperature changes in the bucket. (See fig. A-1) An ignition current is provided by electrodes. The internal pressure can rise to 1500 psig so the oxygen bomb is usually designed to withstand 3000 psig.
Precision is highest when the calorimeter’s jacket temperature is carefully controlled. In a the 6200 calorimeter changes in the temperature are continuously compensated in real time by monitoring the difference in temperature between the bucket and the jacket’s actual temperature.
Figure 1 Theoretical temperature vs. time of a combusted sample in bomb
The purpose of the lab experiment was to learn physical and chemical properties of the three phases fuels can exist in solids, liquids and gaseous; learn more about the principles and methodology for calculating heating values; he relationship between heating value and the thermal efficiency/engine performance; better understand higher heating values (HHV) and lower heating values (LHV); and understand fuel chemistry and how the heating values of liquids and gaseous fuels are different. In the bomb oxygen combusts the carbon and hydrogen and forms carbon dioxide and water and oxidation of any other elements present.
The objective of the lab was to use a semi-automated Parr 6100 Bomb Calorimeter to measure the heating value of diesel fuel (liquid) and of a wood pellet (solid).
1.2 Materials and Equipment
Crucible
Precision balance
60 mesh sieve
Fuse with electrodes
6100 Oxygen Bomb Calorimeter
Oxygen cylinder
Cylinder key
1.3 Methodology
The fuel was placed in a clean, dry crucible and weighed on the precision balance. Solid samples were sieved through a 60 mesh and compressed. The support stand is used to hold the bomb head. The crucible holding the sample was placed in an electrode loop and then fastened with a 10 cm long fuse wire between the two electrodes. Each of the two fuse wires is bent towards the sample without touching the sample or the crucible. The bomb was closed according to the following steps (a) sealing ring was checked and moistened with water, (b) the bomb head was slid into the cylinder and carefully pushed down, (c) the screw cap was turned firmly onto the cylinder top manually, and (d) the gas release nut was also tightened by hand.
Next the oxygen cylinder was opened by the key. The filling hose for oxygen to the bomb inlet valve, the calorimeter control panel key marked O2 fill was pressed. The calorimeter was allowed to warm up for twenty minutes. The bucket was filled with 2 kg of distilled water and then the bucket was put into the calorimeter. The lifting handle of the bucket was inserted in the two holes of the bomb’s screw cap and next lowered into the water. The banana plugs were connected to the bomb head’s terminal sockets afterwards the bomb was covered with water. The wires were moved far away the stirrer. The lifting handle was lifted out of the bucket while shaking excess water back into the bucket. The calorimeter cover was closed. The appropriate IDs were entered into the control panel.
At the end of combustion the bucket was removed so the bomb could be lifted out. A towel was used to dry off the bomb. First the gas release valve was opened very slowly while counting to 1 minute. The pressure having been released the screw cap could be opened so the bomb head could be removed from the cylinder and placed on the support stand. (The head was not twisted.) The inside of the bomb was checked for any evidence of incomplete combustion. The bomb interior was cleaned with distilled water.
1.4 Results
Measurements from the experiment can be found in Table 1. The result is the Calculated Higher Heating Value because the heat of the water vaporization is included.
1.4.1 Calibration of the Calorimetry
The amount of heat introduced by the reference sample (EE)
= Heat of Combustion of the standard material x Weight of the sample burned. In a standardization test/change of temperature
Higher Heating Value (HHV) ==> HHV = EE * ΔT /fuel sample weight
Diesel fuel
Fuel sample weight = 0.606 g diesel, and
∆T = 2.7613◦C
then
EE = (0.606 g) *(6318 cal/g)/ (2.7613◦C) = 1386.56 cal/◦C
EE = 1386.56 cal/◦C * 4.1868 Joules/cal = 5805.25 J/◦C = 5.8 kJ/◦C
HHV = (5.8 kJ/◦C) * (2.7613◦C) / (0.606 g) = 26.43 kJ/g = 26.43 MJ/Kg
Wood Pellet
Fuel sample weight = 0.827 g wood pellet, and
∆T = 1.6022◦C
then
EE = (0.827 g) *(6318 cal/g)/ (1.6022◦C) = 3261.13 cal/◦C
EE = 3261.13 cal/◦C * 4.1868 Joules/cal = 13653.71 J/◦C = 13.6 kJ/◦C
HHV = (13.6 kJ/◦C) * (1.6022◦C) / (0.827 g) = 26.45 kJ/g = 26.45 MJ/Kg
Heating value (HV) is the amount of heat produced when a known quantity of fuel is combusted. For the diesel fuel the calculated HHV was 26.43 MJ/Kg and the obtained value was 45.7805, was almost twice as much. For the wood pellet the calculated HHV was 26.45 MJ/Kg and the obtained value was 19.4116 MJ/Kg, lower than the calculated value. Human error during measurement can causes differences between obtained (experimental) and calculated values of samples. For example the crucible may not have been properly cleaned and dried and/or some of the water was lost from the bucket when the handle was removed. The instrument may not have been given enough time to warm up or the sample had not cooled long enough before the measurements were taken.
The Higher Heating Values (HHV) is equal to the gross heating value. The Lower Heating Value (LHV) is equal to the net heating value. HHV is the amount of heat produced when the combustion products are cooled to the same temperature of the sample before combustion and the water vapor is condensed during combustion. The LHV is calculated when the “latent heat of vaporization of the water vapor formed” is subtracted from HHV (Engineering 2013). HHV can be calculated by adding together the LHV and the latent heat of the vaporization of the water vapor.
Why is the diesel’s heating value higher than the biomass of the wood pellet?
The diesel’s heating value may be higher than the wood pellets because the combustion of the wood was incomplete. Forming the wood pellet from the biomass may have included water vapor. The oxygen could have had impurities that would lead to incomplete combustion.
References
Engineering ToolBox (2013) “Heating Value” [Online] available from <http://www.engineeringtoolbox.com/gross-net-heating-value-d_824.html> [11 Dec 2013]
“Introduction to Bomb Calorimetry.” [Online] No. 483M (2007) Parr Instrument Company, Moline, Illinois, available from http://www.parrinst.com [14 Dec. 3013]
Jessup, R.S. (1960) “Precise Measurement of Heat of Combustion with a Bomb Calorimeter” NBS Monograph 7, U.S. Department of Commerce, National Bureau of Standards, Washington D.C.: U.S. Government Printing Office.
“Parr Oxygen Combustion Calorimeters: Designing and Building High Precision Oxygen Combustion Calorimeters for over 100 Years.” [Online] Bulletin 6000 Calorimeters (2013) Parr Instrument Company, Moline Illinois, available from www.parrinst.com [12 Nov. 2013]
Appendix
Figure 2 Schematic of Bomb Calorimeter
(Introduction 2007: 3)
Lab 2: Ignition Quality/Knock Characteristics of Gasoline and Diesel
1.0 Lab 2: Ignition Quality/Knock Characteristics of Gasoline and Diesel
1.1 Introduction
The less fuel used while at the same time experiencing the highest efficiency possible is a goal of car and truck owners so high ignition quality is desired. Gasoline and diesel fuels have octane and cetane numbers. Octane and cetane numbers are used to identify the type of fuel and measure the efficiency of an engine. In order to save money and fuel when driving a vehicle with a fossil fuel engine the more efficient the operation the more savings. Efficient operation is based on “controlled ignition and combustion of the fuel” (Lewis, Schenk and Stassen 1998). Poor quality fuel can cause a knocking sound in the engine.
An octane/cetane analyzer (meter) can determine the gasoline type and the octane number of gasoline. (See fig. 1-A) The octane level is determined using the Research Octane Number (RON) method and the Motor Octane Number (MON) method. Fuel dielectric permeability is measured to assign numbers to octane and cetane. The sum of polarizability and the dipole moment of lubricating oils is called the dielectric constant (or dielectric permittivity) (Carey 1998).
The laboratory has been carried out to understand how heating values and ignition quality are linked. The experiment also led to increased knowledge of combustion duration, ignition delay and ignition quality. The last objective was to learn why octane and cetane numbers are important. Octane and cetane number measurement standards were learned and how the standards are measured.
1.2 Materials and Equipment
Gasoline
Diesel fuel
Octane/Cetane Meter model SX-100M
1.3 Methodology
In general an Octane/Cetane meter is used by adding sample to the sensor, the meter is turned on and the instrument automatically makes the reading (Shatox 2009).
Gasoline sample
The measuring cartridge was filled with 30 ml of gasoline. The emitter was used to close the cartridge and then the cartridge was connected to the instrument. The detector was switched on and the scroll buttons used put Octane on the screen. The value of ‘100’ appeared. Then the fuel in the emitter was poured out and a new gasoline sample was added (30 ml gasoline). The Octane number was recorded.
Diesel sample
The measuring cartridge was filled with 30 ml of diesel. The emitter was used to close the cartridge and then the cartridge was connected to the instrument. The detector was switched on and the scroll buttons used put Cetane on the screen. The value of ‘47’ appeared. Then the fuel in the emitter was poured out and a new diesel sample was added (30 ml gasoline). The measured reading was recorded. The Cetane number and the temperature were recorded.
1.4 Results
The RON methodology resulted in an octane number of 97.7. The MON methodology resulted in an octane number of 87.7. The average of the resulting octane values using RON and MON methodologies for the petrol sampled was 92.7. The Cetane value measured was 46.8 at a sample temperature of 21.1◦C and freezing point of temperature of -37.2◦C.
Octane is an alkane (C8H18); octane and the isomers are ingredients of gasoline. One of the isomers of octane is 2,2,4-trimethylpentane is also called isooctane. It is used for octane rating as a standard value called the Octane number. Cetane is also known as hexadecane, C16H14. The cetane number is a standard to measure of the detonation of diesel fuel. One of the properties of cetane is that under compression it easily ignites; the Cetane number is 100. Ignition Delay is the parameter measured by the Cetane number; it is the segment of time from the time of the fuel injection until the combustion starts. When the Ignition Delay is known then the Cetane Number can be calculated because of the inverse relationship of the Ignition Delay to the Cetane Number. Engine Knock can be an audible sound heard from the engine. The first step of ignition in the cylinder is fine but when an air/fuel mixture has pockets the mixture explodes without any contact with the spark plugs. Combustion Duration is the time between the start and the end of the reaction of an oxidizing agent and a fuel. An octane meter can identify leaded, unleaded gasoline and gasoline containing additives and the temperature of the gasoline sample. An octane meter can also report a diesel fuel’s cetane number and the freezing temperature of the diesel fuel sample.
1.6 References
Carey, A.A. (1998) “The Dielectric Constant of Lubrication Oils.” [Online] Computational Systems Inc., Knoxville, TN, available from <http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA347479> [11 Dec. 2013]
Engineering ToolBox (2013) “Heating Value” [Online] available from <http://www.engineeringtoolbox.com/gross-net-heating-value-d_824.html> [11 Dec 2013]
Lewis, C.P.G., Schenk, C., and Stassen, W.J.M. (1998) “Ignition Quality of Residual Fuels in Diesel Engines.” [Online] available from <web.anl.gov/PCS/acsfuel//Files/43_1_DALLAS_03-98_0122.pdf> [10 Dec. 2013]
Shatox (2009) “Octane meter SX-100M.” [Online} Shatox Company, available from <http://www.shatox.com/octanemeter.html> [10 Dec. 2013]
Valtadoros, T., Wong, V., and Heywood, J., "Engine Knock Characteristics at the Audible Level." [Online] SAE Technical Paper 910567, 1991, available from doi:10.4271/910567, [10 Dec. 2013]
1.7 Appendix
Figure 1 Portable octane meter (Shatox 2009)
Lab 3: Convergent-Divergent Nozzle Test
1.0 Lab 3: Convergent-Divergent Nozzle Test
1.1 Introduction
The convergent –divergent process is also known as a reversible adiabatic process in the aviation industry. The adiabatic process is the when no transfer of heat or matter passes between a system and its surroundings. The step before the adiabatic process does involve the transfer of heat. The convergent-divergent nozzle test refers to a nozzle that converges closer to the middle of middle of the flow and then opens up to allow more space from the centre of the flow to the walls of the nozzle. (See appendix fig. 1-A) The convergent-divergent nozzle is also known as the De Laval Nozzle.
The system is complex because the speed of the flow is dependent on many variables including pressure, temperature and density. The experimental results were dependent on the pressure ratio of the stagnation pressure to the outlet back pressure. When the ratio is >1 the mass flow rate increases until its maximum value and at this point Mach value is reached at the nozzle’s throat. It is call the choked flow. Supersonic is reached when the pressure ratio is reduced further. After reaching supersonic the flow becomes more complicated and difficult to describe.
The theory used was based on a perfect gas and its reversible and adiabatic flow through a nozzle. This approach made it possible to understand the mass discharge at two points in the nozzle, the inlet stagnation conditions and the overall pressure ratio at the throat. Equation A-1 is a derivation from mRT01AtPo1 . The second equation is derived so the pressure ration can be evaluated in reference to the specific heat capacity ratio (See eqn. A-2).
The objective of the laboratory was to observe and record a convergent-divergent nozzle test for compressible flow and compare the experimental results to theory. The pressure distributions at critical points in the nozzle were observed, recorded and analysed. Research of this type is used for many types of civil and mechanical engineering but the most interesting may be the use in the aerospace sector for designing supersonic aircraft and rockets.
1.2 Materials and Equipment
Apparatus for the convergent-divergent test (See appendix fig 2-A)
Air displacement is caused using a high capacity positive displacement air compressor. An adjustable pressure valve is available to try to reduce fluctuations and reading distortions. The plenum in the apparatus is where the air is held at stagnation pressure and temperature. The Inlet valve controls the air flow into the Plenum. The throttling valve controls the back pressure of the nozzle. The temperature inside the Plenum is measured with a thermometer. The pressure probe measures the pressure at different points in the nozzle. The Orifice plate and manometer measure mass flow rate of air.
1.3 Methodology
The nozzle test apparatus was used for the experiment. The inlet pressure was set to 400 kN/m2. The nozzle outlet valve was completely opened so many readings could be taken as the nozzle outlet valve was slowly closed. And then the outlet valve was shut and allowed to reach 50 kN/m2 higher. Again a full set of data was recorded. The next step required shutting the outlet valve and then increasing the back pressure in 50 kN/m2 increments until the pressure gauge reads >50 kN/m2 below the inlet pressure. Calculations were performed based on a derivation of the ideal gas law and considering the second law of thermodynamics that describes entropy. (See eqns. A-1 and A-2) The data was plotted using the data in Table 1 and in Table A-1.
Figure 1 in the results section show the pressure ratio on the y-axis and the measurement location in the nozzle on the x-axis. The mass flow rates obtained from the different pressure ratios were determined with the Converging-Diverging nozzle test. Theoretically the curve should be smooth and decrease exponentially to show that the back pressure is decreasing with respect to the inlet pressure. The experimental tests 1 through 3 show a fairly good fit to the theoretical until the end closer to where the back pressure is measured. Figure 1 demontrates how Tests 1, 2 and 3 follow an exponential decrease from 10 to 25 but then from 25 to 30 the experimental results do not fit the curve expected for the theoretical. The graph also shows the axial pressure distribution as measurement location versus pressure ratio. The table in the appendix when used to graph Figure 2 demonstrates that the design pressure ratio is from 0.06 to 0.70. (See fig. 2)
The axial pressure distribution was graphed and was determined by the pressure ratio. The flow patterns are similar to those found in the literature. Where the plot lines on the graph are not smooth is a reflection of an error during measurement while changing the pressure by 50 kN/m increments. The theoretical curves would be very smooth as opposed to the irregular curves from the experimental results. The discrepancies could be due to the pressure drop and/or to the 50 kN/m incremental changes in the pressure that are difficult to carry out smoothly in the laboratory. The change of 50 kN/m in pressure could be to large so it would be good to do the experiment implementing a 25 kN/m change in order to have a better linear fit of the experimental to the theoretical curve. Other experimental errors could influence the discrepancies in the theoretical to the experimental values such as the necessity to better calibrate the air compressor. If there were any leaks those would negatively affect the reliability of the data. Taking turns to change the pressure could introduce error biased towards the member of the group in charge of changing the pressure. If mistakes were made reading the barometer meniscus then those would add to error; students unfamiliar with reading the barometer measurements could have possibly made such a mistake.
1.6 References
Devenport, W.J. Engineering applets.[Online] available from <http://www.engapplets.vt.edu/fluids/CDnozzle/cdinfo.html> [11 Dec. 2013]
Lamb, M. and Taylor, J. G. (1996) Static Internal Performance of a Two-Dimensional Convergent-Divergent Nozzle with External Shelf. NASA Technical Memorandum 4719, Langley Research Center, NASA, Hampton, VA,
Stewart, J.D. (1969) Mass flow rate for nearly-free molecular slit flow. [Online] Journal of Fluid Mechanics, (3), pp. 599-6008, available from <http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=382180> [9 Dec. 2013]
1.7 Appendix
Figure A-1 Convergent-divergent (C-D) nozzle
Figure A-2 Schematic of a convergent-divergent nozzle test
The Plenum is the rectangular box with thick black lines.
Equation A-1Ideal gas flow used to quantify flow
Equation A-2 Substitution for pressure ratio
=
Table 1 Data from the convergent-divergent nozzle test (na = not available)
