Lecture 4 - First Law of Thermodynamics PDF

“Thermodynamics ” Chapter 4 First law of thermodynamics Dr. Ahmad Elshamy Dr. Ahmad Elshamy The British University in Egypt Conte nts 01 Objectives 02 Definitions 03 En...

“Thermodynamics ” Chapter 4 First law of thermodynamics Dr. Ahmad Elshamy Dr. Ahmad Elshamy The British University in Egypt Conte nts 01 Objectives 02 Definitions 03 Energy balance 04 First law for closed systems 05 Specific heats 06 First law for open systems 07 Efficiency Dr. Ahmad Elshamy The British University in 0 Egypt 1 The objectives of Chapter 3 are to: Introduce the concept of energy balance Identify the first law of thermodynamics for closed systems. Develop the general energy balance applied to closed systems. Define the specific heat at constant volume and the specific heat at constant Dr. Ahmad Elshamy The British University in 0 Egypt Definitions 2 The first law of thermodynamics The first law of thermodynamics, also known as the conservation of energy principle, provides a sound basis for studying the relationships among the various forms of energy and energy interactions. The first law of thermodynamics states that energy can be neither created nor destroyed during a process; it can only change forms.  Dr. Ahmad Elshamy The British University in 0 Egypt Energy balance 3 The conservation of energy principle can be expressed as follows: The net change (increase or decrease) in the total energy of the system (ΔE) during a process is equal to the difference between the total energy entering and the total energy leaving the system during that process. That is, ( 𝑻𝒐𝒕𝒂𝒍 𝒆𝒏𝒆𝒓𝒈𝒚 () - 𝑻𝒐𝒕𝒂𝒍 𝒆𝒏𝒆𝒓𝒈𝒚 = 𝒆𝒏𝒕𝒆𝒓𝒊𝒏𝒈 𝒕𝒉𝒆 𝒔𝒚𝒔𝒕𝒆𝒎 𝒍𝒆𝒂𝒗𝒊𝒏𝒈 𝒕𝒉𝒆 𝒔𝒚𝒔𝒕𝒆𝒎 )( 𝑪𝒉𝒂𝒏𝒈𝒆𝒊𝒏𝒕 𝒐𝒕𝒂𝒍 𝒆𝒏𝒆𝒓𝒈𝒚 𝒐𝒇 𝒕𝒉𝒆 𝒔𝒚𝒔𝒕𝒆𝒎 ) Ein – Eout = ΔEsystem Dr. Ahmad Elshamy The British University in 0 Egypt Energy balance 3 Energy change of a system, ΔEsystem The determination of the energy change of a system during a process involves the evaluation of the energy of the system at the beginning and at the end of the process. Energy change = Energy at final state – Energy at initial state ΔEsystem = Efinal – Einitial = E2 – E1 systems), the change in the total energy of a system For simple compressible during a process is the sum of the changes in its internal, kinetic, and potential energies and can be expressed as ΔE = ΔU + ΔK.E + ΔP.E Wher ΔU = m(u2- , ΔK.E = ½m(v22-& ΔP.E = mg(z2- e: u 1) v12) z1) Dr. Ahmad Elshamy The British University in 0 Egypt Energy balance 3 Stationary systems Most systems encountered in practice are stationary, that is, they do not involve any changes in their velocity or elevation during a process. Thus, for stationary systems, the changes in kinetic and potential energies are zero (that is, ΔK.E & ΔP.E = Zero), and the total energy change: ΔE = ΔU Also, the energy of a system during a process will change even if only one form of its energy changes while the other forms of energy remain unchanged. Dr. Ahmad Elshamy The British University in 0 Egypt First law for closed systems 4 Energy balance for closed systems Rate form For a closed system undergoing a cycle, the initial and final states are identical, and thus ΔEsystem = E2 - E1 = 0. P Then the energy balance for a cycle simplifies to E in - Eout = 0 or Ein = Eout. Noting that a closed system does not involve any mass flow across its boundaries, the energy balance Qnet = Wnet for a cycle can be expressed in terms of heat and work interactions as v Dr. Ahmad Elshamy The British University in 0 Egypt First law for closed systems 4 General formula for closed system The first law cannot be proven mathematically, but no process in nature is known to have violated the first law, and this should be taken as sufficient proof. So, when performing a general analytical study or solving a problem that involves an unknown heat or work interaction and when ΔE=0, we need to assume a direction for the heat or work interactions, as follows: Q-W=ΔU=m(u2-u1) or q-w=Δu=u2-u1 Dr. Ahmad Elshamy The British University in 0 Egypt First law for closed systems 4 Dr. Ahmad Elshamy The British University in 0 Egypt First law for closed systems 4 Dr. Ahmad Elshamy The British University in 0 Egypt First law for closed systems 4 Dr. Ahmad Elshamy The British University in 0 Egypt First law for closed systems 4 Dr. Ahmad Elshamy The British University in 0 Egypt Specific heats 5 The specific heat is defined as the energy required to raise the temperature of a unit mass of a substance by one degree. In general, this energy depends on how the process is executed. In thermodynamics, we are interested in two kinds of specific heats: specific heat at constant volume cv and specific heat at constant pressure cp. The specific heat at constant pressure cp is always greater than cv because at constant pressure the system is allowed to expand and the energy for this expansion work must also be supplied to the system. kJ/kg C kJ/kg K 3.12 5.19 kJ kJ Dr. Ahmad Elshamy The British University in 0 Egypt Specific heats 5 Internal energy, enthalpy & specific heats of ideal gas The change in internal energy or enthalpy for an ideal gas during a process from state 1 to state 2 is determined by: At low pressures, all real gases approach ideal-gas behaviour, and therefore their specific heats depend on temperature only. Thus, Dr. Ahmad Elshamy The British University in 0 Egypt Specific heats 5 Internal energy, enthalpy & specific heats of ideal gas Dr. Ahmad Elshamy The British University in 0 Egypt Specific heats 5 Internal energy, enthalpy & specific heats of ideal gas Dr. Ahmad Elshamy The British University in 0 Egypt Specific heats 5 Internal energy, enthalpy & specific heats of ideal gas Dr. Ahmad Elshamy The British University in 0 Egypt Specific heats 5 Internal energy, enthalpy & specific heats of solids and liquids Internal energy change: Enthalpy change: Dr. Ahmad Elshamy The British University in 0 Egypt First law for open systems 5 Mass and volume flow rates The amount of mass flowing through a cross section per unit time is called the mass flow rate and is denoted by m ˚ m =ρ A V ˚ c avg (kg/s) Where: ρ is the density of the flowing fluid Ac is the cross-section area the fluid is flowing through Vavg is the average velocity the fluid is flowing with The volume of the fluid flowing through a cross section per unit time is called the volume flow rate, V˚ (m3/s) Dr. Ahmad Elshamy The British University in 0 Egypt First law for open systems 5 Conservation of mass principle The conservation of mass principle for a control volume can be expressed as: The net mass transfer to or from a control volume during a time interval Δt is equal to the net change (increase or decrease) in the total mass within the control volume duringΔt. That is, min - mout = ΔmCV (kg) Σm˚in = Σm˚out (Steady flow) Dr. Ahmad Elshamy The British University in 0 Egypt First law for open systems 5 Flow work Unlike closed systems, control volumes involve mass flow across their boundaries, and some work is required to push the mass into or out of the control volume. This work is known as the flow work, or flow energy, and is necessary for maintaining a continuous W = PV flow(kJ) through or awcontrol = pvvolume. (kJ/kg) flow flow Total energy of flowing fluids As discussed earlier, the total energy of a simple compressible system consists of three parts: internal, kinetic, and potential energies. So, the change of energy for steady flow open system can be expressed as: Ein = Eout (Steady Q˚in+W˚in+mflow) ˚ (h+v2/2+gz)in = Q˚out+W˚out+m˚(h+v2/2+gz)out Dr. Ahmad Elshamy The British University in 0 Egypt First law for open systems 5 Some steady-flow engineering devices Compressor & turbine Nozzle & diffuser Throttling valve & mixing chamber Dr. Ahmad Elshamy The British University in 0 Egypt First law for open systems 5 Nozzle & diffuser Nozzles and diffusers are commonly utilized in jet engines, rockets, spacecraft, and even garden hoses. A nozzle is a device that increases the velocity of a fluid at the expense of pressure. A diffuser is a device that increases the pressure of a fluid by slowing it down. Throttling Compressor & valve & mixing Dr. Ahmad Elshamy The British University in 0 Egypt First law for open systems 5 Compressor & In turbine steam, gas, or hydroelectric power plants, the device that drives the electric generator is the turbine. As the fluid passes through the turbine, work is done against the blades, which are attached to the shaft. As a result, the shaft rotates, and the turbine produces work. Compressors, as well as pumps and fans, are devices used to increase the pressure of a fluid. Work is supplied to these devices from an external source through a rotating shaft. Therefore, compressors involve Throttling Nozzle & work inputs. valve & mixing Dr. Ahmad Elshamy The British University in 0 Egypt First law for open systems 5 Throttling valve & mixing In engineering applications, mixing chamber two streams of fluids is not a rare Throttling valves are any kind of flow-occurrence. The section where the restricting devices that cause a mixing process takes place is significant pressure drop in the fluid.commonly referred to as a mixing Some familiar examples are ordinary chamber. adjustable valves, capillary tubes, and porous plugs. Unlike turbines, they produce a pressure drop without involving any work. The pressure drop in the fluid is often accompanied by a large drop in temperature, and for that reason throttling devices are commonly used in refrigeration Nozzleand & air-conditioningCompressor & applications. turbine Dr. Ahmad Elshamy The British University in 0 Egypt First law for open systems 5 Dr. Ahmad Elshamy The British University in 0 Egypt First law for open systems 5 Dr. Ahmad Elshamy The British University in 0 Egypt First law for open systems 5 Dr. Ahmad Elshamy The British University in 0 Egypt First law for open systems 5  0 Dr. Ahmad Elshamy The British University in Egypt 7 Efficiency: Efficiency (ɳ) indicates how well an energy conversion or transfer process is accomplished. Performance or efficiency, in general, can be expressed in terms of the desired output and the required input. Efficiency (performance) = Desired output / Combustion Required input Electric appliances Mechanical efficiency the ratioefficiency of the useful energy In the efficiency absence of any The actual energy transferred to the energy irreversibilities such as received from burning consumed by the appliance friction, mechanical energy 1 kg of fuel compared can be converted entirely to the heating value. from one mechanical form to ɳelectric app = Energy another ɳcombustion = Q / utilised / Energy ɳmech = Emech.out / HV transferred to Emech.in appliance  0 Dr. Ahmad Elshamy The British University in Egypt 7 Efficiency: Mechanical efficiency Pump, compressor or Turbine efficiency fan efficiency ɳpump = Mechanical energy ɳturbine = Mechanical energy increase of fluid output / Mechanical energy input / Mechanical energy decrease of W˚pump,u / W˚pump fluid W˚turbine / W˚turbine,e Thank you