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

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k k eff s 1 3 2 (4.55) This correlation has been examined and it was found that although simple, it accurately produces very similar results as the other complex correlations presented in the literature. The effective densities and specific heats of the fluid and solid phases are calculated as follows [15]: c f , eff f f s eff s c f , eff f f s , 1 (4.56) c s , eff 1 c (4.57) 4.3 Hydrodynamic analysis 4.3.1 Gas-liquid interaction The countercurrent flow of gas and liquid in a packed column is characterized by two operating points: loading and flooding. The gas–liquid interaction is one of the essential factors affecting pressure drop in countercurrent two phase flow in packed beds. It can be assumed [37] that the increasing velocity difference of the two phases leads to a number of effects on the liquid film. The interface will first become increasingly wavy, after which the gas entrains droplets and finally forces the liquid film upwards. These effects will lead to an increase of the pressure drop, which represents the extra energy dissipation in the gas phase. Therefore, Strigle [36] gives two definitions for the lower loading point, which is the highest flow rate at which the pressure drop is proportional to the square of the gas flow rate, and the loading point, which is the flow rate at which the vapor phase begins to interact with the liquid phase to increase the interfacial area in the packed bed. Above this gas flow rate, the column will reach a maximum capacity that will be determined either by massive liquid entrainment in the gas phase or by excessive liquid hold up in the packed bed. At high liquid flow rate, depending on the size of the packing, the packed bed voids largely tend to be filled with liquid, and some of the gas phase actually is aspirated down the column in the liquid phase. A sufficient liquid retention time in the base of the column will permit these aspirated gas bubbles to rise to the surface of the liquid pool and escape back into the gas phase. Strigle [80] emphasized that prudent designer will use a large enough packing to avoid this area of operation. For example, 5 cm (2 inch) packings don't exhibit this phenomenon below a liquid rate of 88.6 ton.hr -1 .m -2 , and 8.85 cm (3.5 inch) packings have been operated at liquid rates as high as 158 ton.hr -1 .m -2 . The foregoing maximum design liquid rates are based on a liquid viscosity no higher than 1 cps for 2.5 cm size packing, 0.0018 kg.m -1 .s -1 for 3.8 cm size packing, and 0.0032 kg.m -1 .s -1 for 5 cm size packing. 87
At flooding, essentially all the voids are filled with liquid. The flooding point can be described in a number of ways, but the clearest definition [39] is the point where at a certain gas flow, flow reversal of the liquid occurs and the liquid is unable to flow through the packing and collects on the top of the bed. The pressure drop per unit height increases rapidly due to this liquid accumulation. This point is the operating limit for counter-current flow of gas and liquid through solid packing. Flooding is a kind of hydrodynamic phenomenon to be prevented in the operation of packed columns, which are extensively used in absorption and distillation, and in empty tubes mainly used in reflux condensers, where the condensate moves downward counter-currently with respect to the uprising vapor. Kister and Gill [32] applied the principle that the packing pressure drop at flooding point increases as the packing capacity increases to derive a simple flood point correlation. 0.7 PFl 0.115Fp (4.58) Equation (4.58) expresses the pressure drop at the flood point as a function of the packing factor (F p ) alone. It is recommended [32] to use this correlation for flood point prediction if pressure drop data are available or when pressure drop can be reliably predicted. Flooding is not often of significant importance for small schemes with relatively low gas flow rates, such as small scale HDH system proposed in the present study. On contrary, as the HDH systems are usually operated at relatively low liquid loads, the minimum wetting rate becomes significantly more important than the flooding rate. The minimum wetting rate (MWR) is the lower stability limit of packing. It is the liquid load below which the falling liquid film breaks up, and the liquid storage causes dewetting of the packing surface [32]. Kister [32] presented various correlations and commonly known rules of thumb for estimating the MWR and recommends a simple correlation/rule of thumb: L MW 60 a 0. 5 L (4.59) where L is a reference minimum wetting rate based on an average value of a about 60 gpm.ft -2 . Values of L for different packing materials are tabulated in Kister [32], for PVC and Polypropylene L is 1.4 and 1.6 gpm.ft -2 (~ 950.5 – 1086.3 l.m -2 .h -1 ) respectively. 4.3.2 Column Sizing Three different techniques are being used for tower sizing; the flooding point, the maximum operation capacity (MOC), and the maximum pressure drop criteria. In flooding point criterion, packed towers are usually designed at 70 to 80 percent of the flood point flow velocity. This practice provides sufficient margin to allow for 88
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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
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of energy storage or hybridization
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Desalination development focuses on
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Introduction of the indirect type s
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2 Literature Survey 2.1 Introductio
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There is actually a commercial prod
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Q m H c ( T T1) c ( T 2 T ) (2.2)
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(solid-liquid) have a storage densi
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(iii) a sharp melting temperature.
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melt and freeze without segregation
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However, the stored heat is less va
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parameters on the heat transfer and
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categories. The first category is b
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to 550 kWh/m 3 of distilled water a
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The enthalpy and humidity of the sa
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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 76 and 77: packing where it gets in direct con
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 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
Page 126 and 127: The product condensed water was mea
Page 128 and 129: operating limits. In light of the l
Page 130 and 131: In the last two cases, the inlet ho
Page 132 and 133: performance and its threshold value
Page 134 and 135: It could be interpreted that this p
Page 136 and 137: Evaporator Condenser Figure 5.8: Av
Page 138 and 139: within the inaccuracy margins. The
Page 140 and 141: The last two cases of boundary cond
Page 142 and 143: natural air flow in the system. In
Page 144 and 145: 6 Model Implementation and Validati
Page 146 and 147: x t u x u 0 0 , (6.5) and for a s
Page 148 and 149: Next time step validation were the
Page 150 and 151: The evaporator and condenser both w
Page 152 and 153: Figure 6.2: Evolution of inlet and
Page 154 and 155: 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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