numerical analysis of heat and mass transfer in heat and moisture exchanger (hme) pezhman payami 1...
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Numerical Analysis of heat and mass transfer in Heat and Moisture Exchanger (HME)
Pezhman Payami1
Supervisors: Masud Behnia1, Barry Dixon2
1 Fluid Dynamics Group, School of Mechanical Engineering, 2 Saint Vincent’s Hospital, Melbourne
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Contents
› Significance
› General Classification of HMEs
› Problem Specification
› Heat Transfer Mechanisms
› Methodology
› References
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Significance
Normal breathing and nose function
To warm and humidify inspired air in
upper airways to reach the alveoli as
saturated vapour at the core temperature
To maintain core body temperature within
an appropriate range
To prevent drying of the tracheal mucosa
and other structures causing respiratory
mucosal dysfunction and hypothermia Upper airways and nose structure
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Significance
Mechanically ventilated patients
When the upper airways are bypassed by oral or nasal endotracheal intubation it is
essential to seek an alternative way to heat and humidify inspiratory gases
HME is an artificial nose (passive humidifier) that traps expiratory heat and moisture in
a medium and returns a portion of it to the next inspiration
HME as an artificial nose in mechanically ventilated patients
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General Classification of HMEs
HMEs
Hygroscopic
Hydrophobic
Composite (Hygroscopic/Hydrophobic)
Composed of plastic foam, wool or paper condensation surfaces with a low thermal conductivity Impregnated with a hygroscopic chemical such as Calcium Chloride to improve moisture conserving properties
Large pleated surface composed of ceramic fibres Covered by a synthetic resin that repels the water
Felt filter layer such as polypropylene non-woven fibre subjected to an electrical field to improve filtration efficiency Moisture exchange component of polyurethane open-cell foam or cellulose fibre (either cotton or wood pulp) impregnated with Calcium Chloride
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Problem Specification
Patient side
(T=34ºC, RH=100%)
Ventilator side
Peak airway pressure:
less than 30 cmH2O
Flow rate:
30 l/min
Frequency:
12-16 times per minute
Temp and RH:
room air conditions could be assumed for the first run
› The flow is considered incompressible/ steady/ laminar
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Heat Transfer Mechanisms
Conduction
•Inside/between porous material and the casing
Convecti
on
•Between gas phase and solid portion of porous material
•Between the casing and the ambient air
Radiatio
n
•Radiation heat transfer across the domain (could be neglected)
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Methodology
Porous
Flow
Modified N-S equations using Darcy’s law
Linear directional loss defined by streamwise and transverse permeabilities
Heat transfer
Energy equation for solid phase
Interfacial heat transfer between the fluid and solid considering overall heat transfer coefficient between the fluid and the solid
Mass transfer Mass concentration of each component according to ideal gas equation of state
Fluid
Flow N-S equations
Heat transfer
Energy equation for fluid phase by considering porosity effect in the porous zone
Interfacial heat transfer between the fluid and solid considering overall heat transfer coefficient between the fluid and the solid
Mass transfer Mass concentration of each component according to ideal gas equation of state
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Methodology
General transport equation
Where is a general variable that can be replaced with macroscopic properties of the fluid such
as pressure, velocity components or temperature to describe the behavior of the flow
In the porous zone the Darcy’s law is governed by
A computational fluid dynamics package, ANSYS CFX 13, is used to simulate fluid flow and heat
transfer in the HME
Sgraddivdiv
t
u
Rate of increaseof of fluid
element
Net rate of flowof out of
fluid element(convection)
Rate of increaseof due to
diffusion
Rate of increase of due tosources
=+ +
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References
› Tariku F, Kumaran M.K., Fazio P., Transient Model for Coupled Heat, Air and Moisture Transfer Through Multilayered Porous Media, International Journal of Heat and Mass Transfer, 53, pp. 3035-3044, 2010.
› Baggio P., Bonacina C., Schrefler B.A., Some Considerations on Modelling Heat and Mass Transfer in Porous Media, Transport in Porous Media, 28, pp. 233-251, 1997.
› Kaya Ahmet, Aydin Orhan, Dincer Ibrahim, Numerical Modelling of Heat and Mass Transfer During Forced Convection Drying of Rectangular Moist Objects, International Journal of Heat and Mass Transfer, 49, pp. 3094-3103, 2006.
› R. Younsi R., Kocaefe D., Poncsak S., Kocaefe Y., Gastonguay L., CFD Modelling and Experimental Validation of Heat and Mass Transfer in Wood Poles Subjected to High Temperatures: a Conjugate Approach, International Journal of Heat and Mass Transfer, 44, pp. 1497-1509, 2008.
› Eva Barreira, João Delgado, Nuno Ramos and Vasco Freitas (2010). Hygrothermal Numerical Simulation: Application in Moisture Damage Prevention, Numerical Simulations - Examples and Applications in Computational Fluid Dynamics, Lutz Angermann (Ed.), ISBN: 978-953-307-153-4, InTech, Available from: http://www.intechopen.com/articles/show/title/hygrothermal-numerical-simulation-application-in-moisture-damage-prevention
› Dellamonica J., Boisseau N., Goubaux B., Raucoules-Aime M., Comparison of Manufacturers’ Specifications for 44 Types of Heat and Moisture Exchanging Filters, British Journal of Anaesthesia, 93 (4), pp. 532-539, 2004.
› ANSYS, ANSYS CFX-Solver Theory Guide. 2010, Canonsburg, PA