Experimental and Numerical Analysis of a PCM-Supported ...

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packing where it gets in direct contact with the concurrent flow of hot humid air at liquid-gas interface. The air continuously circulates in closed loop cycles, as a carrier medium, to transfer the generated water vapor from the evaporator to the condenser where it is condensed back into highly purified fresh water. 3.3 Concept of Multi-Effects of Heating and Humidification (MEHH) The main physical difference between the two phase flow in the PCM evaporator and condenser investigated in the present study and a conventional packed bed is that the latter is equipped with a nearly non-conductive packing elements. The nonconductive packing usually has a negligible heat capacity. This implies that the energy balance and energy flow in the conventional packed beds are only governed by the interaction between both fluid phases (i.e. hot and cold streams). On the other side, for the PCM (or any other conductive material) as packing media, the solid phase contributes to the energy balance and the interstitial energy flow between fluid phases. In the latter, since there are wetted areas and dry patches, there is simultaneous heat and mass transfer entirely between all phases in both evaporator and condenser. Therefore, the physical problem under investigation involves multiscales; on the pore scale level or the micro scale as well as on the macro scale level or the overall balance which mutually affects the sensible and latent heat components at different interfaces on the micro-scale level. 3.3.1 Micro scale level Existence of PCM packing (or any conductive medium) in the evaporator and condenser creates a parallel path for heat and mass transfer between gas and liquid through the packing elements due to partial wetting of the packing surfaces as shown in figure 3.2. Let the subscript “s” identify the solid particles and subscripts “l” and “g” designate the liquid water and humid air (air-water vapor gas mixture) respectively. Consider a rectangular finite slab which has thermal conductivity k covered on one of its sides by hot liquid water and subjected to ambient air on the other side as shown in figure (3.2b). Due to the higher liquid temperature, there will be two sensible heat transfer components between water and gas (Q lg ) and water and solid (Q ls ). The interaction between gas and solid phases (Q gs ) arises due to the temperature difference between both of them. At the liquid gas interface, the latent heat of vaporization becomes part of the energy balance, and the mass diffusion (m v ) caused by the vapor pressure gradient affects both heat and mass balances. 57
On the other hand, when evaporation depends on the interfacial vapor pressure, the sensible energy flow components impact the film temperature at liquid gas interface and consequently affect the latent heat component. At steady state conditions, the energy flow from liquid to solid phase equilibrates with the energy flow from solid to air and the local temperatures of all phases remain constant. This analysis indicates that thermal conductivity represents one of the key parameters that control local heat and mass transfer not only at solid-liquid and solid-gas interfaces but also at liquid gas interface. a b Figure 3.2: Illustration of local heat and mass transfer flow between different components in the evaporator; (a) one packing element, (b) finite element Figure 3.3: Sketch of the locally established multi-effects of heating and humidification in the evaporator 58
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Technische Universität München In
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It is my pleasure to thank Prof. Th
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Contents List of figures ..........
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5.3 Functional description ........
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List of Figures Figure 1.1: Cost br
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78cm; Top: Evaporator, Bottom: Cond
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xii
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m Mass flow rate; [kg.s -1 ] m eva
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ed c coll cond cw d d e or eff evap
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1 Introduction The first part of th
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summarized in figure (1.2). Thermal
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1.3.2 Selection criteria of desalin
Page 26 and 27: of energy storage or hybridization
Page 28 and 29: Desalination development focuses on
Page 30 and 31: Introduction of the indirect type s
Page 32 and 33: 2 Literature Survey 2.1 Introductio
Page 34 and 35: There is actually a commercial prod
Page 36 and 37: Q m H c ( T T1) c ( T 2 T ) (2.2)
Page 38 and 39: (solid-liquid) have a storage densi
Page 40 and 41: (iii) a sharp melting temperature.
Page 42 and 43: melt and freeze without segregation
Page 44 and 45: However, the stored heat is less va
Page 46 and 47: parameters on the heat transfer and
Page 48 and 49: categories. The first category is b
Page 50 and 51: to 550 kWh/m 3 of distilled water a
Page 52 and 53: The enthalpy and humidity of the sa
Page 54 and 55: Air mass flow rate [kg/h] GOR [-] 1
Page 56 and 57: On the other hand, the efficiency o
Page 58 and 59: 2.8.4.2 Effect of air to water mass
Page 60 and 61: distillate rate. The system was not
Page 62 and 63: were; EnviPac Gr.1, 32 mm, and VFF
Page 64 and 65: Figure 2.15: Height versus air temp
Page 66 and 67: The performance of the condenser is
Page 68 and 69: Klausner and Mei [136] described a
Page 70 and 71: The impact of the collector cost ca
Page 72 and 73: exchanger and energy storage in one
Page 74 and 75: 3 Scope of the Study This chapter p
Page 78 and 79: In fact there is a tradeoff between
Page 80 and 81: The energy flow between all and eac
Page 82 and 83: 4 Theoretical Analysis and Modeling
Page 84 and 85: The two-energy equation models whic
Page 86 and 87: solid-liquid phase change inside th
Page 88 and 89: assumed by conduction in both axial
Page 90 and 91: the air pressure drop ∆P per unit
Page 92 and 93: evaporation rate per unit volume of
Page 94 and 95: where β t , β c , ρ ref , c ref
Page 96 and 97: the viscous transport and introduce
Page 98 and 99: liquid and solid phases, respective
Page 100 and 101: 4.2.3.3 Liquid and gas hold-ups Liq
Page 102 and 103: Several models and approaches for m
Page 104 and 105: where G=ρU d /ε g is the pore mas
Page 106 and 107: k k eff s 1 3 2 (4.55) This cor
Page 108 and 109: uncertainties associated with the f
Page 110 and 111: 1 2 (4.62) d1 capp c2 c1 c2 1
Page 112 and 113: [58] evaluated the cooling tower ai
Page 114 and 115: 1 exp NTU 1 C r, cond cond (4
Page 116 and 117: A HE QHE U T HE LM (4.81) Where Q
Page 118 and 119: c M f b (4.86) M b S S f where r
Page 120 and 121: that form the study logical foundat
Page 122 and 123: 5 Experimental Analysis This chapte
Page 124 and 125: 5.2 Measuring Techniques K-type the
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The product condensed water was mea
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operating limits. In light of the l
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In the last two cases, the inlet ho
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performance and its threshold value
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It could be interpreted that this p
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Evaporator Condenser Figure 5.8: Av
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within the inaccuracy margins. The
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The last two cases of boundary cond
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natural air flow in the system. In
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6 Model Implementation and Validati
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x t u x u 0 0 , (6.5) and for a s
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Next time step validation were the
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The evaporator and condenser both w
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Figure 6.2: Evolution of inlet and
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Figure 6.4: Evolution of inlet and
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temperature is at its maximum level
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Bottom Top Figure 6.7: Evolution of
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7 Parameters Analysis of the Evapor
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Figure 7.1: Effect of packing size
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e 0.45 kPa under natural convection
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aspect ratio under constant bed vol
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Figure 7.7: Effect of inlet hot wat
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well as the time required to reach
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Figure 7.10: Effect of packing medi
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40°C respectively. This gives flex
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the condensate rate, productivity f
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esult, rather than PCM media which
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Figure 7.18: Effect of PCM thermal
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8 Optimization of HDH plant In this
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The MATLAB model describes how the
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less steep than that of the distill
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8.3.4 Effect of hot water flow rate
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storage materials and thus it can h
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for latent heat storage would be ob
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When T m becomes sufficiently highe
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9 Summary and conclusions 9.1 Summa
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productivities at any cooling water
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[13] Vafai K., Sozen M. An investig
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[48] Crone S., Bergins C., and Stra
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[79] Tiwari G.N. and Yadav Y.P. (78
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[109] Mehling H., Hiebler S. and Zi
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[139] Belessiotis V, Delyannis E (2
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Appendices Appendix A A1. PCM Therm
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A3. Constants and Thermo-physical p
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From the above correlation, the con
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The Ergun analogy factor J h repres
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Equation (C.7) represents an overal
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Appendix D D1. Cooling tower theory
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A m w = the plan or cross sectiona
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