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Modeling and engineering a conditioning terminal: a CFD approach to thermal comfort in housesModutech S.r.l.October 28 2014

CAE Conference

International CAE ConferenceOctober 28th 2014Ing. Alessandro CarianiModutech S.r.l.

11

Cooling houses means handling not only generators but even heat terminals as fan coils or brand new philosophy fan radiators as soon as integrated cooling systems. This analisys has the aim to present a detailed CFD study of the transient forced laminar convective heat transfer over a complex mixed radiative/convective/forced convective system, when the thermal field is due to different kinds of variations in time and space of some boundary conditions, i.e. plate temperature or wall heat flux. The governing equations are solved using extensions either of the differential method, or the Karman Pohlhausen integral approach. Artistic approach is the first part of integration of R&D to find the state of the art of technology, comfort and design: CFD is the basis of a brand new product.Abstract

HEAT TRANSMISSION PHENOMENANatural convectionHeat RadiationForced convection

Natural convection

The natural convection flow field is a self-sustained flow generated by the presence of a temperature gradient. As a result of this gradient, we obtain a density gradient of flux, typically air. A surface with a difference in temperature will induce a flow current with the influence of the gravitational field and the density field gradient.Room air heating, wall heating and turbulence inside the room is one of the results of these phenomena THERMAL POWER Q, transmitted by convection between a surface and a surrounding fluid, can be calculated using Newton's law:QC = S hc (Ts - T) Natural convection

Heat amount transmitted depends on:Surface geometry;Surface orientation;Difference of temperature between air and surface;Surface roughness

Air temperature behave in a quite similar way in free and forced convection, while air relative speed tends to increase up to a distance of 3 mm from the surface of a plate and then again tend to zeroS = heat exchange surface (m2)h = coefficient of convective heat exchange (W/m2 K)Natural convectionQC = S hc (Ts - T)

Natural convectionAmong the types of convective transfers, forced convection is often used because of its efficiency. Aa soon as the natural convection has the advantage to be free in terms of energy expense it generates low heat transfer coefficient. Thus it will be interesting to improve free convection heat transfer, by the mean of time-dependent boundary conditions.Laminar free convection problem on a vertical wall has been plentifully investigated as the dynamic behaviour of free convection flows is poorly documented in literature.These CFD analisys of mixed flow behaviuor indicates that:Radiative systems could increase overall efficiency using concrete wall to cooperate with other heat flux phenomena;Control of natural and forced convection can increase the perceived comfort of users.

Natural convectionIn natural convection, the fluid motion occurs by natural means such as buoyancy. Since the fluid velocity associated with natural convection is relatively low, the heat transfer coefficient encountered in natural convection is also low. Consider a hot object exposed to cold air. The temperature of the outside of the object will drop (as a result of heat transfer with cold air), and the temperature of adjacent air to the object will rise. Consequently, the object is surrounded with a thin layer of warmer air and heat will be transferred from this layer to the outer layers of air. The temperature of the air adjacent to the hot object is higher, thus its density is lower. As a result, the heated air rises. This movement is called the natural convection current. Note that in the absence of this movement, heat transfer would be by conduction only and its rate would be much lower. In a gravitational field, there is a net force that pushes a light fluid placed in a heavier fluid upwards. This force is called the buoyancy force.

Natural convectionNote that the net force is proportional to the difference in the densities of the fluid and the body. This is known as Archimedes principle. We all encounter the feeling of weight loss in water which is caused by the buoyancy force. Other examples are hot balloon rising, and the chimney effect. Note that the buoyancy force needs the gravity field, thus in space (where no gravity exists) the buoyancy effects does not exist. Density is a function of temperature, the variation of density of a fluid with temperature at constant pressure can be expressed in terms of the volume expansion coefficient , defined as:

It can be shown that for an ideal gas:

where T is the absolute temperature. Note that the parameter T represents the fraction of volume change of a fluid that corresponds to a temperature change T at constant pressure. Since the buoyancy force is proportional to the density difference, the larger the temperature difference between the fluid and the body, the larger the buoyancy force will be. Whenever two bodies in contact move relative to each other, a friction force develops at the contact surface in the direction opposite to that of the motion. Under steady conditions, the air flow rate driven by buoyancy is established by balancing the buoyancy force with the frictional force.

Forced convectionConvection is the mechanism of heat transfer through a fluid in the presence of bulk fluid motion. As in natural convection the fluid motion is caused by natural means such as the buoyancy effect, in forced convection, the fluid is forced to flow over a surface or in a tube by external power. Study of convective heat transfer is one of the most complicated problem in fluid-dynamics since it involves fluid motion as well as heat conduction between solids (typically plates of heat sinks) and fluid: turbulent flows increase the effects of fluid heat transfer (higher is the flow speed the higher is heat transfer rate).Convection rate heat transfer can be expressed by Newtons law of cooling:

The convective heat transfer coefficient h strongly depends on the fluid properties and roughness ( of the solid surface, and the type of the fluid flow (laminar or turbulent).

Forced convectionIt is assumed that the velocity of the fluid is zero at the wall, this assumption is called noslip condition. As a result, the heat transfer from the solid surface to the fluid layer adjacent to the surface is by pure conduction, since the fluid is motionless. Thus,

The convection heat transfer coefficient, in general, varies along the flow direction. The mean or average convection heat transfer coefficient for a surface is determined by (properly) averaging the local heat transfer coefficient over the entire surface. Bigger is roughness of plate, lower is heat transmission, as soon as higher is Prandlt number better heat propagates due to bigger sublayer.

Volume controlForced convection

Forced convection

Forced convection

Radiation is the transfer of energy (heat) between two throught electromagnetic waves.Instead of conduction and convection, radiation does not need direct contact between exchangers, and does not require a medium to propagate throught.

Qr = S T4THERMAL POWER Q, transmitted by radiation can be calculated using Boltzmann's law:Heat radiation

= 5,67 x 10-8 W/m2 K4 Boltzmann constant materialemissivity Polished gold0,02Copper tube0,30Polished steel0,17Water 0,96

S = heat exchange surfaceHeat radiation

The main parameters for the efficiency of the heat exchange both free convective that irradiation are the exchange surface and the thermal jump: greater is surface greater is the heat input; greater is thermal jump greater is the radiation heat output.Heat radiation

quadratic trend of the thermal radiancelinear trend of temperature rise in free convection

Overall convection

Fluid dynamic simulation

Overall heat

Why use CFD in cooling ?Analysis and Design1. Simulation-based design instead of build & testMore cost effective and faster than EFDCFD provides high-fidelity database for diagnosing flow field2. Simulation of physical fluid phenomena that are difficult for experimentsFull scale simulationsEnvironmental effects (wind, weather, etc.)Simulation in case of different living conditions (party ?)

ModelingModeling is the mathematical physics problem formulation in terms of a continuous initial boundary value problem (IBVP)IBVP is in the form of Partial Differential Equations (PDEs) with appropriate boundary conditions and initial conditions.Modeling includes: 1. Geometry and domain 2. Coordinates 3. Governing equations 4. Flow conditions 5. Initial and boundary conditions 6. Selection of models for different applications

Modeling (geometry and domain)Simple geometries can be easily created by few geometric parameters Complex geometries must be created by the partial differential equations or importing the database of the geometry(e.g. airfoil) into commercial softwareDomain: size and shape Typical approaches Geometry approximationCAD/CAE integration: use of industry standards such as Parasolid, ACIS, STEP, or IGES, etc.The three coordinates: Cartesian system (x,y,z), cylindrical system (r, , z), and spherical system(r, , ) should be appropriately chosen for a better resolution of the geometryEffect: mesh analisys in commercial software MUST be checked before CFD run: garbage in, garbage out.

MeshMeshes should be well designed to resolve important flow features which are dependent upon flow condition parameters (e.g., Re), such as the grid refinement inside the wall boundary layerMesh can be generated by either commercial codes (Gridgen, Gambit, etc.) or research code (using algebraic vs. PDE based, conformal mapping, etc.). A check is always needed !!The mesh, together with the boundary conditions need to be exported from commercial software in a certain format that can be recognized by the research CFD code or other commercial CFD software.

Solve Setup appropriate numerical parametersChoose appropriate Solvers Solution procedure (e.g. incompressible flows) Solve the momentum, pressure Poisson equations and get flow field quantities, such as velocity, turbulence intensity, pressure and integral quantities (lift, drag forces)

xyz

xyz

xyz

(r,,z)

z

r

(r,,)

r

(x,y,z)CartesianCylindricalSpherical

General Curvilinear Coordinates

General orthogonal Coordinates

Navier-Stokes equations (3D in Cartesian coordinates)

ConvectionPiezometric pressure gradientViscous terms

Local accelerationContinuity equationEquation of stateRayleigh Equation

CFD Process How to proceedViscous ModelBoundary ConditionsInitial ConditionsConvergent LimitContoursPrecisions(single/double)Numerical SchemeVectorsStreamlinesVerificationGeometry

Select Geometry

Geometry ParametersPhysicsMeshSolve Post-ProcessingCompressibleON/OFFFlow propertiesUnstructure(automatic/manual)Steady/UnsteadyForces ReportXY Plot

Domain Shape and SizeHeat Transfer ON/OFFStructured(automatic/manual)Iterations/StepsValidationReports

Check mesh

consider a system made by a fan and a radiator into a room

Boundary Conditions:

Inlet mass flow;Outlet mass flow;Real wall (T=293,2 K);Rotating region;Fluid Subdomain:AirWaterRadiative Surface;Solid materialsCFD simulation of the MDTCH/1

CFD simulation: room positioning

TEMPERATURECFD simulation

VELOCITY

CFD simulation

31

VELOCITYCFD simulation

32

Flow Simulation

AnalysisReportGases: AirPath: Gases Pre-DefinedSpecific heat ratio (Cp/Cv): 1,399Molecular mass: 0,0290 kg/molDynamic viscosityLiquids: WaterPath: Liquids Pre-DefinedDensityOptimization

CFD room simulation.

Example of CFD simulation on a room with big glasses and one cooling terminal

CFD analisys resultsMixed heat phenomena in cooling terminals can assure better performances if compared to single heat transfer technologies as the efficiency of heating behavior changes depending on plate, air and heat exchange fluid temperature;Natural convection is low energy depending thanks to buoyancy effect, but heat rate is low: as soon as the request of fast heating is present (i,.e. in hotels rooms) this process need a continuous heat power to heat fluid to be appreciated by users;Forced convection is medium energy depending thanks to blower generated air speed, and heat rate is high: as soon as the request of fast heating is present (i,.e. in hotels rooms) this process need a continuous blower power to be appreciated by users;Radiative power phenomena is low when heat exchange fluid does not reach approximately 50 celsius (see Boltzmann equation): radiative heat is perceived as nice by users when surface is big, so heating of wall must be an important part of heating processes.Mixed cooling process can assure a perfect flexibility in cooling rooms adding air parameters controls.

CFD analisys results:

Temperature change (medium value) inside a room heated at different heat fluid temperature

CFD analisys results:

Room stratigraphy temperature inside a room Natural convection and radiation on a cooling terminal

CFD analisys results: MDTCH/1 at different power

Heat (J)Time (s)

Comfort ?

Relative moisture index (%)

Comfort ?

Heat power (kW)Delta temperature of heat exchangers surface and air (Kelvin)

And at the end, the art of cooling systems

Modeling and engineering a conditioning terminal: a CFD approach to thermal comfort in houses

Thank youModutech S.r.l.October 28 th 2014

CAE Conference

International CAE ConferenceOctober 28th 2014Ing. Alessandro CarianiModutech S.r.l.

4848