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hole is 0.65 mm (0.00065 m). The bias vent flow rate can be calculated by using following equation [43, 44]: m 2 C ( A n ) P sign( P ) (2.27) BV Ma d BH BH M M Where C d is the discharge coefficient for each bias vent hole, A BH is the smallest cross sectional area in each hole and n BH is the number of bias vent holes. using PM sign( PM ) in the equation instead of simply PM attributes to the fact that if the patient’s inhalation is too strong and the tube flow can not provide enough air to fill the mask, pressure in the mask might be lower than the ambient pressure and air around the mask might enter the mask through these holes. Figure 2.19 The shape of bias vent holes This axial cross-sectional picture (Figure 2.19) shows that the bias vent holes are somewhat cone-shaped. Considering shrinkage of plastic after moulding, the sharp angles may become rounded so the shape of each hole is similar to a Venturi tube. A typical value of 0.985 as Venturi tube’s discharge coefficient is chosen for these bias vent holes [45]. 28
Inserting Eq. (2.26) and Eq. (2.27) into Eq. (2.25) and rearranging gives an expression for calculating the fluctuating flow velocity in HADT: 1 M aVMa dPM uT [ m P 0.985 2 Ma ( ABH nBH ) PM sign( PM )] (2.28) A R T dt Ta Tcs ugc Ma 2.4 Reverse flow and air transportation 2.4.1 Reverse flow calculation Reverse flow occurs when the volumetric flow rate of the patient’s exhalation is greater than the bias vent flow rate so the pressure in the mask increases. When it is higher than that in the humidifier chamber, some of the air will be pushed back into the HADT. Reverse flow can be calculated by integrating the possible negative part of the HADT velocity in each breath cycle: DRF uT dt (2.29) Where uT 0 , and integration range is repetitive within n tc t ( n 1) tc for each breath cycle while n 0,1,2,... Flow and reverse flow in the HADT is simplified by assuming that they displace each other. Although mixing near the interface of fresh air and exhaled air produces a gradient of concentration, it is not expected to significantly influence the condensation and CO 2 in the re-breath. 2.4.2 Varying transport delay analysis For dynamic and reverse flow situations, tube air flow velocity is fluctuating with time and can be negative when reverse flow occurs. That is to say, within a breath cycle, air is mainly flowing from the chamber towards the mask then the patient. However, for a short fraction of time within the cycle, air is pushed back by exhalation from the mask towards the chamber. For analysing and mimicking the humid air, the HADT is divided into 30 lumps with each lump of 5.75 cm (1.725 m/30). Due to the varying airflow velocity, time for air flowing over one lump’s length is also fluctuating. This fluctuating time may be derived as follow: 29
Page 1 and 2:
The Effects of CPAP Tube Reverse Fl
Page 3 and 4:
Acknowledgement First of all, I wou
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When deep breathing induced reverse
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2.4.1 Reverse flow calculation ....
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5.3.1 Thermal dynamic model under s
Page 11 and 12: XI.3 Steady state mask thermal bala
Page 13 and 14: List of Figures Figure 1.1 Obstruct
Page 15 and 16: Figure 5.10 Comparison between expe
Page 17 and 18: Figure 6.13 Fluctuation of airflow
Page 19 and 20: Table 5.3 Comparison of condensatio
Page 21 and 22: Nomenclature Symbol Meaning of the
Page 23 and 24: CMFeO CMFet C O C t Concentration o
Page 25 and 26: m C 23 m Ca m cv m Cw m store m i m
Page 27 and 28: Nu Cp Nu dir Nu Ti Nu To Nuwsn Nuws
Page 29 and 30: QMor QMwst QTastn QTicn QTin Q
Page 31 and 32: R Mic R Moc R Ticn R Toc R Torn Mas
Page 33 and 34: V Capacity of the container m 3 V C
Page 35 and 36: 1.1 Background Chapter 1 Introducti
Page 37 and 38: The MRD is worn in user’s mouth w
Page 39 and 40: Figure 1.4 CPAP machine Figure 1.5
Page 41 and 42: 1.3 Literature survey A literature
Page 43 and 44: CPAP fluid dynamic performance with
Page 45 and 46: Chapter 2 Mathematical Modelling of
Page 47 and 48: 2.3 Fluid dynamic analysis This sec
Page 49 and 50: If there are minor pressure drops,
Page 51 and 52: Figure 2.6 Pressure sensor - Honeyw
Page 53 and 54: The pressure readings were taken af
Page 55 and 56: Figure 2.12 Experimental set up for
Page 57 and 58: 2.3.4 Chamber air space mass balanc
Page 59 and 60: From the above analysis, the pressu
Page 61: Figure 2.18 Full face mask and the
Page 65 and 66: Where n( ) t is the airflow proper
Page 67 and 68: However in reality, mixing turns ou
Page 69 and 70: Chapter 3 Mathematical Modelling of
Page 71 and 72: An Environment Control Chamber (Vö
Page 73 and 74: Figure 3.4 Thermal enthalpy gain of
Page 75 and 76: Figure 3.6 Heating element beneath
Page 77 and 78: Figure 3.8 Chamber water heat balan
Page 79 and 80: Table 3.1 Thermal resistance from h
Page 81 and 82: 3.3.1.2 Heat balance at the outer s
Page 83 and 84: 3.3.2 Heat flow from chamber water
Page 85 and 86: through ADU which is a function of
Page 87 and 88: It is assumed that the direct impac
Page 89 and 90: Sc a a (3.47) Dwa Analogous to th
Page 91 and 92: and keep them in gaseous status. On
Page 93 and 94: through the walls and natural conve
Page 95 and 96: Where d Ca is the specific humidity
Page 97 and 98: TTa ( n1) is also considered as inl
Page 99 and 100: R Torn 2 2 ATlor ( TTWn T )( TTWn
Page 101 and 102: condensation severity but cannot ca
Page 103 and 104: condensation has occurred at all or
Page 105 and 106: The inlet is the air from HADT duri
Page 107 and 108: 3.9 Average temperature of inhaled
Page 109 and 110: Table 4.1 Inputs to the fluid dynam
Page 111 and 112: Figure 4.2 Simulink TM model for fl
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Table 4.4 Outputs from the overall
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Gain block after input 1 is to conv
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Figure 4.7 Varying Transport Delay
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4.2.5 Mask mixing calculation subsy
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4.3 Computational model of thermal
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Figure 4.11 Block diagram of CPAP t
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Figure 4.13 Simulink TM model for t
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The outputs are listed in Table 4.1
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Figure 4.16 Dynamic chamber-air the
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Figure 4.17 HADT lump thermal balan
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Figure 4.18 Steady state HADT lump
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Figure 4.19 HADT lump air dynamic f
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Figure 4.21Steady state mask full t
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4 Temperature of airflow from HADT
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Average evaporation rate subsystem
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HADT and connects those upstream se
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The experiment was also carried out
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air flowing through the elbow, a ce
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expansion from the elbow to the mas
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Figure 5.12 Environmental control r
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Table 5.1 shows the combinations of
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Figure 5.17 Comparison of model out
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The comparison of experimental resu
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5.3.1.4 Airflow temperature at the
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flow rate had been even further inc
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Figure 5.31 Comparison of condensat
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Table 5.3 Comparison of condensatio
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3. The corrugated grooves on the HA
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5.3.2.1 Validation of evaporation r
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The experiment observation showed t
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Figure 6.1 Reverse flow under diffe
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It can also be seen in Figure 6.2 t
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within the same period. However, wh
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Figure 6.6 Comparison of evaporatio
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Figure 6.8 Airflow velocity in HADT
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Figure 6.11 Airflow velocity in HAD
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In Figure 6.14 the blue curve repre
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6.3.3.1.2 Comparison when heating e
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This vaporization potentiality (coi
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Figure 6.20 In-tube condensation of
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When using different sized masks, t
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Figure 6.23 Average specific humidi
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The average specific humidity in an
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Table 6.5 Comparison of average spe
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6. The deep breathing and reverse f
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Appendices Appendix I Regression of
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Appendix I. Regression of the press
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Figure I. 1 Tested results and the
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Appendix II. Regression of the pres
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Table III. 2 Airflow temperature an
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Appendix IV. The corrugated HADT ou
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The total area of the complicated c
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Figure V. 1 Regression of saturated
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This ratio can be regressed against
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Appendix VII. Details of the CPAP f
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VII.4 The mask pressure subsystem F
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Inputs to this water-centred subsys
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Input port number Input Unit 1 Heat
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input 4 is the portion of impact ar
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VIII.2.3 Water surface mass transfe
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This subsystem also contains two su
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VIII.2.5 Chamber wall 1 heat transf
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Figure VIII. 13 Wall 1 outer surfac
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Wall 2 and 3 inner surface temperat
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Figure VIII. 17 Chamber wall 2 and
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Inputs to this subsystem are listed
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Appendix IX. Details of steady stat
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Figure IX. 3 The HADT lump wall out
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Appendix X. Details of HADT lump ai
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2 Absolute value of airflow velocit
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Figure X. 5 HADT lump inlet absolut
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Figure XI. 2 Steady state mask mixi
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Figure XI. 4 Steady state mask wall
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Input port number Input Unit 1 Aver
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Appendix XII. Details of mask air d
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4 Length of the triangular plate in
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8 Length of the triangular plate in
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Appendix XIII. Details of the auxil
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XIII.3 Breath load average subsyste
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Figure XIII. 7 Average evaporation
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XIV.1.2 Under normal ambient temper
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XIV.2.3 Under high ambient temperat
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XIV.4 Airflow temperature at the en
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XIV.5.2 Under normal ambient temper
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XIV.6.3 Under high ambient temperat
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Appendix XVI. Regression of kinetic
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Figure XVII. 2 Condensation/evapora
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Figure XVIII. 2 Condensation/evapor
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at point C. Right after that, here
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Appendix XIX. Coefficient and param
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Average water thermal conductivity
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Appendix XXI. Fluid dynamic and the
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The lung simulator drives the air.
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Appendix XXII. Model user instructi
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The input blocks and their values a
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Dynamically fluctuating air tempera
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References [1] Good sleep advice. W
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[28] Schettino GPP, Chatmongkolchar
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[60] Marek R, Straub J. Analysis of
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