THERMODYNAMICS I INTRODUCTION TO NAVAL ENGINEERING

ENERGY AND HEAT TRANSFER INTRODUCTION TO NAVAL ENGINEERING

THERMODYNAMICS: The science concerned with the inter-relationship between thermal energy and mechanical energy

WHAT ARE THE TYPES OF ENERGY? Stored –Potential (based on position) –Kinetic (based on velocity) Transitional –Energy that is in the process of being transferred from one object or system to another. All energy in transition begins and ends as stored energy

HOW CAN KINETIC ENERGY BE STORED ENERGY? Definition of energy –“the ability to produce an effect” The Bullet example –Has the capability for an effect –But it needs something to hit in order to transfer that energy (the effect)

MECHANICAL ENERGY Potential Energy (PE) – PE = mgh Kinetic Energy (KE) – KE = (1/2)mV 2 Mechanical Energy in TRANSITION: –Called Work –Wk = FD

THERMAL ENERGY l Stored Thermal Energy: Called Internal –Internal Potential Energy Associated with the force of attraction that exist between molecules. –Internal Kinetic Energy Associated primarily with the activity of the molecules Thermal Energy in TRANSITION: –Called Heat –Requires a temperature difference between two systems

MEASURING THERMAL ENERGY l In the real world we use CALORIES and JOULES but... l BRITISH THERMAL UNIT –Quantity of Heat required to raise the temperature of 1 lbm of water from 50.9F to 60.9F –Please don’t write that down

MODES OF HEAT TRANSFER

(1) CONDUCTION Heat flows from hotter to colder region when there is physical contact between the regions

CONDUCTION (cont.) The total quantity of heat passed –is proportional to the cross-sectional area of the conductor over a given time –is proportional to the time of heat flow –in a given length of time is proportional to the thermal gradient (temp difference) –depends on the thermal conductivity of the substances

GENERAL CONDUCTION EQUATION Q = kTA (t 1 -t 2 )/L where Q: Quantity of heat (Btu or cal) k: Coefficient of thermal conductivity (Btu/((hr)(F)(ft)) T: Time (hr) t 1 : Temp at hot end (F) t 2 : Temp at cold end (F) L: Distance between the two ends (in) A: Cross sectional area (sq ft)

CONDUCTION EXAMPLE HOT COLD LENGTH AREA

(2) RADIATION Mode of heat transfer that does not involve any physical contact between the emitting and receiving regions

(3) CONVECTION The mechanical transportation of a mass of fluid from one place to another –Beyond the molecular level –Movement of fluid within fluid –Transportation, not transfer –Fluid’s thermal energy remains in stored form unless it is transferred by radiation or conduction

CONVECTION (cont) TWO TYPES OF CONVECTION –Natural Occurs when there are differences in the density of different parts of the fluid. The difference in density are usually caused by a temperature difference. –Forced Occurs when some mechanical device, such as a pump or a fan, produces movement

DEFINITIONS l Sensible Heat –When heat added results in the change in temperature (kinetic energy) l Latent Heat –When the heat added results in a physical change of the substance (potential energy) Saturation Temperature/Pressure –Psat/Tsat –The point at which liquid and vapor may exist in equilibrium contact with each other

DEFINITIONS (cont) l Saturated Liquid/Vapor –A liquid/vapor at a specified pressure which is at Tsat for the pressure –“Wet vapor” l Subcooled Liquid –A liquid at that specified pressure which is below the Tsat l Superheated Vapor –A vapor that has been raised above Tsat for a given pressure

DEFINITIONS (cont) l Latent Heat of Vaporization –Amount of heat necessary to change a mass of liquid to vapor without changing the temperature l Latent Heat of Fusion –Amount of heat that must be added/removed to a unit mass to melt/solidify it

GENERAL CONDUCTION EQUATION REVISITED Q = kTA (t 1 -t 2 )/L MAXIMIZE MINIMIZE

Counterflow Heat Exchanger Parallel Heat Exchanger TYPES OF HEAT EXCHANGERS

Crossflow Heat Exchanger TYPES OF HEAT EXCHANGERS

FACTORS FOR “K” l THE BOUNDARY LAYER l SCALE/CHEMICAL DEPOSITS l SOOT/DIRT BUILDUP