Design for Manufacture. 3. Thermodynamics

Transcription

1 Desig for Maufacture 3. hermodyamics

2 hermodyamics hermodyamics study of heat related matter i motio. Major developmets durig Egieerig thermodyamics maily cocered with work producig or utilisig i machies such as - Egies, - urbies ad - Compressors together with the workig substaces used i the machies. Workig substace fluids: capable of deformatio, eergy trasfer Air ad steam are commo workig substace Desig ad Maufacture

3 ressure F A Force per uit area. Uit: ascal (a) N / m Bar 0 5 a stadard atmosphere.035 Bar Bar si (poud force / square ich, N 0.48 poud force) si a Desig ad Maufacture

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. Desig for Maufacture

5 rocess A substace is udergoe a process if the state is chaged by operatio carried out o it Isothermal process - Costat temperature process Isobaric process - Costat pressure process Isometric process or isochoric process - Costat volume process Adiabatic rocess No heat is trasferred, if a process happes so quickly that there is o time to trasfer heat, or the system is very well isulated from its surroudigs. olytropic process Occurs with a iterchage of both heat ad work betwee the system ad its surroudigs E.g. Noadiabatic expasio or compressio Desig for Maufacture

6 Eergy, Work ad ower Eergy - Capacity of doig work Work - A force is moved through a distace I a pisto, if pressure is costat work doe A L AL ( ) Nm (Joule) I variable pressure case work doe d L ower - Rate of doig work J/s Watt Desig for Maufacture

7 Work doe i polytropic process workdoe d C C + d ( ) + + Desig ad Maufacture

8 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 he heat liberated by burig uit mass or volume of a fuel. e.g. petrol: 43MJ/kg

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

10 he Coservatio of Eergy For a system Iitial Eergy + Eergy Eterig Fial Eergy +Eergy Leavig

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

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

13 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 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.

14 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.

15 Gas laws Boyle s law (66) For perfect gas C -costat emperature remais costat. or - costat (Isothermal process) Charle s Law (work by 780, Gay-Lussac 80 published) costat - costat (Isobaric process) Gay-Lussac'ss law ( Actually by Guillaume Amotos 700) costat -- costat (Isometric process)

16 Combied gas law (834 by Clapeyro, 856 by Kroig, 857 by Causius) Let c c c, c ad c 3 c 3 or c c c 3 a (costat) or Let mra, where m is the mass, R - specific gas costat (air: R87 J/kgK) mr - characteristic equatio of gas (adiabatic with surroudigs)

17 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 t pressure c p U + ) ( ) U ( mc p

18 olytropic rocess C Whe, isothermal process; p Whe γ v c p c /, it is a adiabatic process From d / ad

19 From the gas characteristic equatio ad / / i.e.

20 Ethalpy - o describe total eergy i gas H U + U iteral eergy pressure v volume Specific Ethalpy h U/m u + v Desig for Maufacture

21 Etropy (absolute temperature) Area heat trasferred s s s(etropy) Etropy is a quatity s that associates with the temperature ad heat trasfer Qrev i the followig way: ΔQrev Δs dqrev ds or dqrev ds where Q rev - reversible heat trasfer: Q rev s s ds Etropy describes the availability of thermal eergy. A isolated system ca oly chage to states of equal or greater etropy.