Energy and Heat Transfer

Objectives Comprehend Forms of energy Energy conversion Heat transfer processes Principles of operation of various heat exchangers

What is Thermodynamics? The science concerned with the interrelationship between thermal energy and mechanical energy i.e. how to convert Thermal Energy to Mechanical Energy

ENERGY

HEAT TRANSFER Science that deals with methods by which thermal energy is able to move from one location to another

THREE MODES OF HEAT TRANSFER CONDUCTION RADIATION CONVECTION

CONDUCTION Heat flows from hotter to colder region when there is physical contact between the two regions EX: Touching a hot pot of water

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)

RADIATION Heat transfer via infrared electromagnetic waves (infrared - wavelength just below visible light) Infrared radiation passes through transparent substances (air, glass, space) without warming them to any significant extent EX: Thermal radiation from sun warms earth w/o warming space in between

Radiation Heat transfer - Hot Cold NoNo physical contact between regions Heat Sink: –Absorbs radiant energy (Dark/Opaque) –Reflects radiant energy (White/Shiny/Polished) Ex. Sun warms the earth

CONVECTION The transportation or movement of some portions of a FLUID within a larger mass of fluid This results in mixing within the fluid As mixing takes place, heat transfer occurs from one part of the fluid to another and between fluid and its surroundings EX: Air Conditioner and fan

Convection Combined conduction/radiation Heating of adjacent molecules Occurs in liquids and gases Types –Natural –Forced

TYPES OF CONVECTION NATURAL CONVECTION: Density differences (usually caused by differences in temperature) within different parts of a fluid create flow FORCED CONVECTION: Use of a mechanical device (pump, fan) to produce movement of the fluid mass

HEAT EXCHANGER (HX) A device that takes thermal energy from one substance and adds it to another substance EX: Car radiator

HX CLASSIFICATION (1) Relative direction of fluid: parallel, counter, cross-flow (2) Number of passes the fluid makes (3) type of contact between fluids: direct (they mix) or indirect (4) Phases of the working substances

Heat Exchangers Counter flow Parallel flow Cross flow Single pass Multi-Pass

Counter Flow Counter Flow

Parallel Flow Parallel Flow

Cross Flow Cross Flow

Main Condenser

Heat Exchangers Single pass - One fluid passes the other fluid only once. Multi-Pass - One fluid passes the other more than once via the arrangement of the tubes, inlet/outlet ports, or by baffles used to guide a fluid through a specific path.

Boundary Layers Boundary Layer - Thin layer of stagnant fluid adjacent to heat exchanger wall. Poor at transferring heat.

Boundary Layers (cont) Effect on heat transfer –Velocity –Turbulent Flow –Large temperature gradients Scale/Chemical deposits Soot/Dirt buildup

BOUNDARY LAYER - The relatively stagnant layers between fluids in convection - sometimes called fluid films

Thermodynamic Cycles Def’n: a recurring series of thermodynamic processes through which an effect is produced by transformation or redistribution of energy One classification: –Open: working fluid taken in, used, & discarded –Closed: working medium never leaves cycle, except through leakage; medium undergoes state changes & returns to original state

Five Basic Elements of all Cycles Working substance: transports energy within system Heat source: supplies heat to the working medium Engine: device that converts the thermal energy of the medium into work –Heated: heat added in engine itself –Unheated: heat received in some device separate from engine

Five Basic Elements of all Cycles Heat sink/receiver: absorbs heat from the working medium Pump: moves the working medium from the low-pressure side to the high-pressure side of the cycle

Basic Thermodynamic Cycle HEAT SOURCE HEAT SINK Pump EngineW Q in Q out Working Substance

THE SECOND LAW OF THERMODYNAMICS

Clausius Statement It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower temperature body to a higher temperature body Heat flows only from a hotter to a colder substance

Entropy Entropy is the theoretical measure of energy that cannot be transformed into mechanical work in a thermodynamic system; entropy may be viewed as an index of the unavailability of energy (energy that is not converted to useful work) The second law of thermodynamics may also be stated as: No process can occur in which the entropy of an isolated system decreases; the total entropy of an isolated system can theoretically remain constant in some reversible (ideal) processes, but in all irreversible (real) processes the total entropy of an isolated system must increase

Reversible v. Irreversible A reversible thermodynamic process –could be made to occur in precisely reverse order, so that the energy system and all associated systems would be returned from their final condition to the conditions that existed before the process started –could return all energy that was transformed or redistributed during the process from its final to its original form, amount and location ALL REAL PROCESSES ARE IRREVERSIBLE TO SOME DEGREE, AND THEREFORE INVOLVE AN INCREASE IN ENTROPY