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3.6 HADT wall inner surface condensation analysis One of the objectives of this thesis is to analyse and predict whether and where along the HADT condensation will occur under a certain condition and setting combination and to compare such condensation between steady flow and breath-added situations. There are three types of condensation namely direct contact condensation, homogenous condensation and surface condensation [58]. The direct contact condensation does not apply to this CPAP machine. For the condensation in the HADT, when the tube heating is off, the airflow temperature inside the HADT is higher than the HADT wall temperature, so the surface condensation will always occur prior to the homogenous condensation. When the tube heating is on, usually there will be no condensation. Even there is homogenous condensation, it will be hard for condensate to form on the HADT wall inner surface because of entrainment. Therefore, it is reasonable to only consider surface condensation in this thesis. 3.6.1 General comparison of condensation and evaporation When moist air flows over a surface, if its humidity level is lower than the saturated humidity level at surface temperature, the air will take in water molecules from the surface. In another words, this airflow has a potentiality of vaporization. Evaporation rate on such a surface is determined by the difference of absolute humidity level in the moist airflow and the Absolute humidity level saturated at surface temperature as well as mass convection coefficient and surface area as mentioned before: m k A( C C ) (3.81) ev ev sv av On the other hand, if the moist airflow touches a surface and the surface temperature is lower than the air’s dew point, some of the water will condense on the surface. That is to say condensation is converse to evaporation [52]. From Marek [60], under same conditions, condensation rate is about 1.2 times of evaporation rate. This means it is practical to use the same evaporation rate calculation equation to calculate both evaporation and condensation if the conditions for evaporation and condensation do not vary significantly. However, it is necessary to point out that the calculation of condensation based on evaporation equation can predict where the condensation will occur and compare the 66
condensation severity but cannot calculate the actual amount of condensate built-up in a period of time. This is because once condensate starts to build up, the thermal conditions on the surface changes and the condensation rate will be no longer the same [42]. 3.6.2 Condensation within the HADT When there is no patient’s breathing, the CPAP machine is working in a steady state and provides a steady flow. The condensation in the HADT can be predicted by comparing the dew point temperature of the airflow to the local HADT wall (the local lump) temperature. If the wall temperature is lower than the airflow dew point, condensation will occur on the inner surface of the wall. By data regression between 5°C ~60°C [61], dew point can be expressed as below (see Appendix V for regression details): T 17.222ln d 93.343( C) (3.82) DewPt Where d is specific humidity defined in chapter 3. The condensation can also be predicted by comparing relative humidity of the airflow in the HADT to the saturated relative humidity at the local HADT wall temperature. If the airflow relative humidity is higher than the wall temperature saturated relative humidity, there will be water vapour spared out from the airflow onto the HADT wall. With the patient’s breathing, the humidity of airflow in the HADT is fluctuating. The surface temperature may be higher or lower than the dew point. The condensation under such fluctuation can only be calculated by humidity comparison. Net condensation within a breath cycle can be used to predict the condensation build-up. The evaporation rate in a lump of the HADT may be expressed as: m k A ( C C ) (3.83) ev ev Tli Tsv Tav Where CTsv is the saturated absolute humidity level at the local HADT wall temperature and CTav is the absolute humidity of airflow through the lump. Analogous to heat convection, the mass convection coefficient k ev may be expressed as: 67
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The Effects of CPAP Tube Reverse Fl
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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
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XI.3 Steady state mask thermal bala
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List of Figures Figure 1.1 Obstruct
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Figure 5.10 Comparison between expe
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Figure 6.13 Fluctuation of airflow
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Table 5.3 Comparison of condensatio
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Nomenclature Symbol Meaning of the
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CMFeO CMFet C O C t Concentration o
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m C 23 m Ca m cv m Cw m store m i m
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Nu Cp Nu dir Nu Ti Nu To Nuwsn Nuws
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QMor QMwst QTastn QTicn QTin Q
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R Mic R Moc R Ticn R Toc R Torn Mas
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V Capacity of the container m 3 V C
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1.1 Background Chapter 1 Introducti
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The MRD is worn in user’s mouth w
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Figure 1.4 CPAP machine Figure 1.5
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1.3 Literature survey A literature
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CPAP fluid dynamic performance with
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Chapter 2 Mathematical Modelling of
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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 and 62: Figure 2.18 Full face mask and the
Page 63 and 64: Inserting Eq. (2.26) and Eq. (2.27)
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: R Torn 2 2 ATlor ( TTWn T )( TTWn
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
Page 113 and 114: Table 4.4 Outputs from the overall
Page 115 and 116: Gain block after input 1 is to conv
Page 117 and 118: Figure 4.7 Varying Transport Delay
Page 119 and 120: 4.2.5 Mask mixing calculation subsy
Page 121 and 122: 4.3 Computational model of thermal
Page 123 and 124: Figure 4.11 Block diagram of CPAP t
Page 125 and 126: Figure 4.13 Simulink TM model for t
Page 127 and 128: The outputs are listed in Table 4.1
Page 131 and 132: Figure 4.16 Dynamic chamber-air the
Page 133 and 134: Figure 4.17 HADT lump thermal balan
Page 135 and 136: Figure 4.18 Steady state HADT lump
Page 137 and 138: Figure 4.19 HADT lump air dynamic f
Page 141 and 142: Figure 4.21Steady state mask full t
Page 143 and 144: 4 Temperature of airflow from HADT
Page 145 and 146: Average evaporation rate subsystem
Page 147 and 148: HADT and connects those upstream se
Page 149 and 150: 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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