Thermodynamics (Revision 1)

Transcription

1 hermodyamics (Revisio ) hermodyamics study of heat related to matter i motio. Egieerig thermodyamics maily cocered with work producig or utilisig machies such as egies, turbies ad compressors together with the workig substaces used i the machies. Workig substace fluids: capable of deformatio, eergy trasfer, commo: air ad steam

2 ressure: F A Force per uit area. Uit: ascal (a) N / m 5 bar 0 a stadard atmosphere.035 bar bar psi (poud force / square ich, N 0.48 poud force) psi a

3 sia vs sig a- absolute g- gauge psia psig + atmosphere pressure e.g: 5 psig psia 5 psig a 7368 a(gauge) 7368 a +035 a( atmosphere) a (absolute)

4 hase Nature of substace. Matter ca exists i three phases: solid, liquid ad gas Cycle If a substace udergoes a series of processes ad retur to its origial state, the it is said to have bee take through a cycle.

5 rocess A substace is udergoe a process if the state is chaged by operatio havig bee carried out o it Costat temperature process Isothermal process Costat pressure process Isobaric process Costat volume process Isometric process or isochoric process

6 Eergy Capacity of doig work Work A force is moved through a distace e.g. I a pisto, work doe A L AL ( ) Uit of work: Nm J (joule) ower Rate of doig work J/s Watt L

7 I geeral case, work work doe d

8 olytropic rocess C - costat a law for the geeral case of expasio or compressio - the idex of expasio or compressio, or a polytropic expoet. E.g. air.4 his is the geeral case for gas.

9 ( ) C d C d workdoe Work doe i polytropic process

10 Heat emperature t (Celsius) (Kelvi) Q heat eergy joules/kg Specific heat capacity: heat trasfer per uit temperature: c dq dt Uit: joules/kg K (joules per kg per K) Calorific value: the heat liberated by burig uit mass or volume of a fuel. e.g. petrol: 43MJ/kg

11 Ethalpy h u + v u iteral eergy

12 riciple of the thermodyamic egie Q Q-W Source Egie Sik W

13 hermal efficiecy η Work doe Heat received W Q Heat Egies A egie i which trasfers eergy results from differece i temperature.

14 Mechaical ower Work doe ower ime ake Uit: J/s watt Electrical ower W I Uit: J/sWatt

15 he Coservatio of Eergy For a system Iitial Eergy + Eergy Eterig Fial Eergy +Eergy Leavig otetial eergy gz Kietic eergy mc

16 Laws of hermodyamics he zeroth Law If body A ad B are i thermal equilibrium, ad A ad C are i thermal equilibrium, the, B ad C must i thermal equilibrium. he first law WQ Meas if some work W is coverted to heat Q or some heat Q is coverted to work W, WQ. It does ot mea all work ca covert to heat i a particular process.

17 he secod law Nature heat trasfer will occur dow a temperature gradiet. he third law At the absolute zero of temperature the etropy of a perfect crystal of a substace is zero.

18 Boyle s law C -costat emperature remais costat. For perfect gas - costat (Isothermal process) Charle s Law - costat (Isobaric process)

19 Combie two processes a (costat) Let mra, where m is the mass, R - specific gas costat (air: R87 J/kgK) mr - characteristic equatio of gas

20 Joule s law Iteral eergy of gas is the fuctio of temperature oly ad idepedet of chages i volume ad pressure. he specific heat capacity at costat volume c v ( ) U v U mc (chage i iteral eergy) he specific heat capacity at costat pressure c p U + ) ( ) U ( mc p

21 olytropic rocess C From ad /

22 From the characteristice equatio ad / i.e.