Energy Transfer and Thermodynamics - Assignment Example

1. Definition of the four laws of thermodynamics The first law of thermodynamics states that energy is neither created nor destroyed but it can be transformed. It further explains that in all processes, the sum of energy in the universe remains unchanged. Therefore, although there are constant energy transformations taking place in the universe, the total energy in a closed system stays the same. The second law of thermodynamics states says that the entropy of a closed system, which is not in equilibrium tends to rise with time and reaches the optimum value when at equilibrium. The third law of thermodynamics says that as temperature tends toward the absolute zero, the entropy of any system will tend towards a stable minimum. The Zeroth law of thermodynamics can be defined in the expression, when system A and system B are both in thermal equilibrium with system C, then also system A is in thermal equilibrium with system B; where A, B and C represent different thermodynamic bodies or system. 2) ii) In general, entropy is simply a measure of a system’s disorder. In thermodynamics, however, entropy is a measure of the amount of energy that is unavailable to do work in a system. ii) When ice melts into water, the water molecules which had been held together breaks apart and are free to move about. In this state, therefore, water is in liquid form. iii) When ice melts into water, the entropy of water and ice increases while that of the surrounding decreases. 3) The entropy of : 2 NO(g) = 2 * 90.3 KJ/mol = 180.6 KJ/mol O2(g) = 0 KJ/mol N2O4(g) = 9.2 KJ/mol i) ΔS in 2 NO(g) + O2(g) →N2O4(g) = [∑ of S in N2O4(g)] – [∑ of S in 2 NO(g) + O2(g)] = (180.6 KJ/mol) - (9.2 KJ/mol + 0 KJ/mol) = 171.4 KJ/mol ii) The entropy decreases since three molecules of gas in the reactants reduces to two molecules of gas. iii) The reaction will be spontaneous 4) i) absolute Zero in, Kelvin is 0 °K Celsius is -273.15 °C Fahrenheit is -459.67 °F Rankine scales is 0 °R ii) The boiling point of water in Kelvin is 373.15 K iii) if the temperature of a system rises by 30°C during a heating process, the rise in temperature in Kelvins (°K ) = °C + 273.15 = 30°C + 273.15 = 303.15 iv) if the temperature of a system rises by 60°F during a heating process, this rise in temperature in, °R = °F + 459.67 = 60°F + 459.67 = 519.67 °R °K = (°F + 459.67) × 5/9 = (60°F + 459.67) * 5/9 = 288.7056 °K °C = (°F − 32) /1.8 = (60°F – 32) / 1.8 = 10 °C 5) When a closed system is in equilibrium, then there is no work that is derived. 6) An example of :- i) An equilibrium state is an ideal gas with a distribution function stabilised to a particular Maxwell-Boltzmann distribution. ii) A steady state can be the fluid flowing through a tube such that the flow is constant. iii) A uniform state is Give examples of equilibrium state, steady state and uniform. (3 marks) 7) A rechargeable battery, a household refrigerator, and a radiator represent closed systems since only energy flows in and out but matter does not. 8) The difference between a gas, a liquid and a solid is that molecules in gas molecules are too loose and therefore, its volume and shape is not definite; a liquid has a definite volume but its shape is nor definite; while a solid has both definite volume and shape. 9) In regards to heat transfer, thermodynamics tell us that there is a driving force, which is the temperature difference, and helps us determine the amount of transferred heat. 10) The difference between internal energy (u) and enthalpy (h): Internal energy is energy within molecules making up a system; while enthalpy is the sum of internal energy and flow work, or rather energy needed to cause a unit of fluid to move in and out of system. . 11) The heat release rate if the mass flow rate is 4kg/s, and the heat of combustion for C3H8 is 46450kJ/kg. Heat release rate = 46450kj/kg / 4kg/s = 11,612.5 kj/s 12) i) Fourier’s Law is the rate equation, which helps in determining flux in conduction heat given temperature distribution within a medium; it state that the time rate of transfer of heat via a medium is proportional to a negative temperature gradient and to the area, at right-angles to this gradient, where heat flows through. ii) Thermal conductivity is a property of matter that denotes the ability to transmit heat. iii) Thermal conductivity of metals is higher than that of insulating material and gases, while insulating material thermal conductivity is higher than that of gases. 13) i) Stefen-Boltzman Law states that total energy radiating from a black-body per unit surface area per second unit time is directly proportional to the fourth power of the absolute temperature. It explains the relation between the total emission of radiation energy by an object and its temperature. ii) Emissivity is the proportion of energy emmited by a specific material to the energy emitted by a black body within the same temperature. iii) The role of view factor in determining the rate of heat transfer is to parameterize the thermal power fraction, when thermal power leaves object A and reaches object B. iv) A blackbody is an ideal object, which can absorb every electromagnetic radiation falling on it. 14) i) Newton's Law of Cooling says that the rate at which an object’s temperature changes is proportional to the difference in the object temperature and that of the surrounding. Therefore, the rate at which an object temperature changes will increase with increase with increasing difference of the two temperatures. ii) Heat transfer coefficient is the amount of heat passing via a unit area of a material per unit time if difference in temperature the system boundaries is one degree. iii) Nusselt number is the proportion of the convective heat transfer to the conductive heat transfer across (normal to) the boundary iv) The two types of convection are natural and forced convection 15) i) Heat of combustion is the amount of heat energy released when 1 mole of a substance (or compound) reacts completely with oxygen. ii) Heat release rate is the amount of heat energy released from an unit area of object per unit time. Example of a reaction iii) A combustion reaction is a reaction involving oxygen with production of energy and flame. Example of a reaction is CxHx+O2→H2O(g)+CO2(g). iv) The two type of combustion reaction are: a): complete combustion where sufficient amount of oxygen is available and results in carbon dioxide and water; and b) incomplete reaction where oxygen is limited and produces water and carbon dioxide as well as carbon and carbon monoxide. v) a) Specific heat capacity is the amount of heat energy necessary for increasing the temperature of unit mass of a substance by one degree. b) Latent heat is energy that is absorbed or released by a substance when it changes its state. c) Calorimetry is a the science that involves measurement of heat in physical changes or chemical reactions. d) Combustion temperature is the temperature existing within a combustion chamber usually expressed in kelvins e) Chemical equilibrium is a state whereby concentrations or chemical reactions of products and reactants do not produce a net change within a period of time. 16) The efficiency of an engine that produces 150J of work from 212J of energy Efficiency = 212j / 150j = 1.4133 17) In a Styrofoam cup (of negligible heat capacity) contains 150g of water at 10°C, and you add 100g of water at a temperature of 85°C; the final temperature of the mixture after it has been thoroughly mixed is: 85°C + 10°C = 95 C 18) A closed Styrofoam cup, which is 6 mm thick and has a surface area of 390 cm2, contains 550 ml of hot coffee at 850C. The air outside the cup remains at a constant temperature of 210C. Assuming that the coffee has a mass of 0.6 kg, and that the specific heat capacity is the same as that of water, (i) The initial rate of heat flow through the Styrofoam (in SI units) is ∆Q/∆t = k * A * ∆T/x = 0.6 * 390 cm2 (850- 210) / 0.6 = 0.6 * 390 cm2 * 640 / 0.6 = 249,600 j/s (ii) The time required for the coffee to cool from 850C to 700C is ∆Q = mc ∆T = 600g * 4.186 joule/gram/°C * 150°C = 376,740 j ∆t = ∆Q/ (A *k) * x / ∆T = 376,740 j / (390 cm2 * 0.6) * 0.6cm = 347.76 s 19) i) Flame is the visible part of fire ii) The different types of flames: a) Premixed flames have oxidizer and fuel mixed prior to occurrence of combustion zone; b) Diffusion has wax as the fuel and air as oxidizer, and do not mix prior to combustion zone; it They are either luminar if the streamlines are smooth without significant bouncing around and turbulent if the flow streamlines are turbulent. 20) i) A fluid is any substance that continuously flows (or deforms) when shear stress is applied. ii) Viscosity of a fluid is the amount of resistance of a fluid to deform under shear stress 21) Air is a compressible fluid while water is an incompressible fluid; this is because velocity divergent of water is zero while that of an ideal gas is above zero . 22) A concrete slab that has a length of 24 m at -8 ºC on a winter's day. When the temperature is 35 ºC and the linear expansion coefficient of concrete is 1 X 10-5 ºC-1, the change in length (∆L) from winter to summer is; ∆L = L0 * αL(T2 − T1) where ∆L is change in length, L0 is original length, T2 and T1 is temperature 2 and 1 respectively, αL is linear expansion coefficient. ∆L = 24 * 10-5 ºC-1 * 43) = 0.1032 m 23) If a box is pushed 5m across a room with a force of 30N, i) The work done (Work = Force * Displacement) = 30 N * 5 m = 150 j ii) The energy used (Work is change in Kinetic energy) = 150j 24) if a piece of aluminium siding is 12.45 meters long on a cold winter's day, -18°C; on a very hot summer's day, 37°C it will be more longer (∆L) by, ∆L = L0 * αL(T2 − T1) and α for aluminium is 23*10-6/K ∆LA = 12.45 m * 23*10-6 *55 = 0.01574925 m; So it is 0.01574925 metres longer 25) i) Boundary layers is a fluid layer at the immediate surrounding with the surface bounding the fluid. ii) A velocity profile for a fluid in a pipe showing both laminar and turbulent flow. References Enrico, Fermi, Thermodynamics, Dover Publications, New York, 1956. McCarthy, Rose, The Laws of Thermodynamics: Understanding Heat and Energy Transfers, The Rosen Publishing Group, New York, 2005. Narayanan, K. V, A Textbook of Chemical Engineering Thermodynamics, PHI Learning Pvt. Ltd., United Kingdom, 2004 Rock, Peter A, Chemical Thermodynamics, University Science Books, United States, 2003 Read More