ME235Lecture%204 - Thermodynamics Lecture 4: Work and Heat...

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1 Thermodynamics Lecture 4: Work and Heat (Chapters 4 & 5) Today's objective: 1. Continue our discussion of: Heat – different modes of heat transfer. 2. Introduce: + + First Law of Thermodynamics Thus, total energy increase or decrease inside the system is: = Q – W Modes of Heat Transfer Conduction: Heat transferred as a result of direct contact. •E x a m p l e s ? ( ) W dT kA Q = ± k – thermal conductivity ~ 100 W/mK for metals, ~1-10 for non-metallic solids & liquids, ~0.1 for insulating materials, & 0.1 to 0.01 for gases. Convection: dx If the medium is flowing (like air or water) heat transfer is enhanced. Examples? coeff. transfer heat convective h ; W Δ T A h Q = = ±
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2 Modes of Heat Transfer Convection: •h ( W/m 2 K ) – value depends on rate of flow and fluid properties. Radiation: The most important mode of heat transfer ( the reason for life on earth ) The most difficult & hence the most ignored. Extremely simplified formula: ( ) ( ) 4 2 8 4 4 / 10 67 . 5 . ; K m W const Boltzmann Stefan W T T A Q surr s × = = σ ε ± 3500 4000 4500 5000 /m2) Convection Radiation 1500 2000 2500 3000 Heat Transfer Rate (W/ 0 500 1000 280 330 380 430 480 530 580 630 680 Temperature (K)
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3 -15 C o Windshield 2 C o T = ? Warm air When you drive a car on a winter day with the atmospheric air at 15 ° C, you keep the outside front windshield surface temperature at +2 ° C by blowing hot air on the inside surface. If the windshield is 0.5 m 2 and the outside convection coefficient is 250 W/m 2 K find the rate of heat loss through the front windshield. For that heat transfer rate and a 5 mm thick glass with k = 1.25 W/m K what is then the inside windshield surface temperature? temperature? Solution: The heat transfer from the inside must = the loss on the outer surface to give a steady state (frost free) outside surface temperature. . Q conv = h A ΔΤ = 250 × 0.5 × [ 2 ( 15)] = 250 × 0.5 × 17 = 2125 W This is a substantialamount of power = the heat transfer from the inside This is a substantial amount of power the heat transfer from the inside . Q cond = k A Δ T Δ x ΔΤ = . Q kA Δ x ΔΤ = 2125 W 1.25 W/mK × 0.5 m 2 0.005 m = 17 K T in = T out + Δ T = 2 + 17 = 19 ° C Similarities between heat and work 1. Heat & work are phenomena that occur at the system boundary. 2 Bthh t d k th 2. Both heat and work are path dependent phenomena 3. The system does not possess heat & work it possesses energy . 4. The consequence of heat & Q is taken positiv Closed Control mass work crossing the system boundary is to change the system state or its energy content. in is taken positive W out is taken positive Thus, total energy ( E ) increase (or decrease) inside the system is: = Q – W
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4 Differences between heat and work ¾ In molecular terms, the chaotic motion of molecules is called thermal motion. The thermal motion of the molecules in the hot surroundings stimulates the molecules in the cooler system to move more vigorously and, as a result, the energy of the system is increased.
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ME235Lecture%204 - Thermodynamics Lecture 4: Work and Heat...

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