Nemli havadaki su buharı yoğuşmasının sayısal olarak incelenmesi
Numerical investigation of water vapour condensation from humid air
Yazar: CİHAT GÜNAY
Danışman: PROF. DR. ALİ FERİDUN ÖZGÜÇ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Isı-Akışkan Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Film yoğuşma = Film condensation

Özet:
Yoğuşma prosesinin analizi birçok mühendislik uygulamasında büyük öneme sahiptir. Özellikle nemli havadaki su buharının yoğuşması soğutma ve iklimlendirme uygulamalarında sık karşılaşılan bir konudur. Bu konu üzerine birçok çalışma mevcuttur. Bu çalışma kapsamında CFD çözücüsü FLUENT kullanılarak nemli havanın yoğuşması sayısal olarak modellenmiştir.
Paralel iki levhadan oluşan bir kanalda, hava akışının analizinde iki boyutlu model geometrisi kullanılmıştır. Laminer akış koşulunda zamandan bağımsız olarak momentum, enerji ve kütle geçişi incelenmiştir. Farklı eleman sayılarına sahip ağ yapıları karşılaştırılarak uygun ağ yapısına karar verilmiştir. FLUENT yazılımının ısı ve kütle geçişiyle ilgili modelleri aktive edilmiştir. Ayrıca yüzeyde yoğuşan su buharı miktarı kütlesel kaynak terimleriyle kontrol edilmiştir. Bu analizde ihtiyaç duyulan bağıntıları kullanmak ve istenilen sonuçları elde etmek için C programlama diliyle yazılan UDF kodları hazırlanmıştır.
Analizlerde hava giriş koşulları değiştirilerek bağıl nem, hız ve sıcaklık parametrelerinin ısı ve kütle geçişi üzerindeki etkileri incelenmiştir. Yapılan analizlerde bağıl nem artışının duyulur ısı üzerinde önemli bir etki yaratmazken gizli ısıyı önemli ölçüde etkilediği görülmüştür. Hava hızı değiştirilerek yapılan analizlerde hız değerinin artmasıyla derişiklik ve ısıl sınır tabaka kalınlıklarının azaldığı gözlemlenmiştir. Ayrıca sıcaklık ve derişiklik dağılımları grafiklerde gösterilmiştir. Sıcaklık değeri değişiminin baz alındığı analizlerde gizli ve duyulur ısı geçişinin sıcaklık değişimiyle aynı yönde olduğu görülmüştür. Ayrıca sıcaklık değişiminden gizli ısı geçişinin duyulur ısı geçişine oranla daha fazla etkilendiği gözlemlenmiştir.
Düz ıslak bir yüzey üzerinden tam gelişmiş akış için ısı ve kütle geçişini veren mevcut korelasyonlar kullanılarak analitik sonuçlar elde edilmiş ve sayısal analizden elde edilen sonuçlarla karşılaştırılmıştır. Yapılan analizlerde giriş parametrelerinin değişiminden bağımsız olarak sayısal çözümden elde edilen kapasitelerin analitik çözümde elde edilen değerlerden yaklaşık oranda saptığı görülmüştür. Toplam 9 koşulda yapılan analizlerin ortalaması alındığında bu oran duyulur ısıda %12 gizli ısıda %16 değerindedir. Bu aradaki oranın oluşmasında sayısal çözümlerde akışın gelişimi göz önüne alınırken mevcut korelasyonların tam gelişmiş akış için olması etkili olmuştur. Her iki çözüm için ısı ve kütle geçiş katsayılarının akış yönündeki değişimi incelenmiştir. Sayısal çözümde ısı ve kütle geçiş katsayılarının giriş kesitine yakın bölgede önemli oranda yüksek olması bu durumu doğrulamaktadır.

Abstract:
Condensation process has great importance in many engineering applications. Especially, modelling of water vapour condensation is fundamental topic in refrigeration and air conditioning systems. In most situations, condensation takes place in the presence of some amounts of noncondensable gases. Theories and models have become more accurate and are now applicable to a wider range of conditions due to condensation phenomenon and its applications have been investigated for many years. In this study, water vapour condensation from humid air was simulated by using CFD solver FLUENT. The main goal was to develop the CFD modelling for the water vapour condensation and to evaluate the accuracy of the condensation model using the source terms were compared with empirical correlations. Both analytical and numerical methods have been used in order to find heat and mass transfer rates are dependent on the humid air parameters like for example the relative humidity, velocity and temperature. 
Condensation is a thermodynamic process of phase transition from gaseous into liquid phase. The process is initiated and sustained when the temperature of surface is maintained below the dew point temperature of surrounding vapour. Condensation may take in one of two modes depending on the condition of the surface. In film condensation, liquid film covers the entire condensing surface which is dominant form of condensation. Otherwise, droplets can occur and slide downward after they attain a certain size if surface is coated with substance that inhibits wetting. Although dropwise condensation is associated with the higher heat transfer rates, equipment are generally designed on the basis of filmwise condensation. Because of dropwise condensation is difficult to achieve and maintain commercially. Therefore, only film condensation was considered in this study. 
The physical model concerned in this study is a horizontal channel. A 2D geometry model was used to carry out the simulation of laminar air flow between parallel plates. The humid air enters the channel with a temperature, velocity and vapour concentration. Plates of the channel were subjected to a uniform temperature and vapour concentration. For all cases, plate temperature was determined according to dew point temperature of humid air and surface concentration was decided according to specific humidity value of saturated humid air at surface temperature. Furthermore, symmetry boundary condition could be applied in vertical direction based on gravity effect was neglected. When using this type of boundary condition in such regions, there is no need for additional boundaries. There is no convective flux across asymmetry plane, and the normal velocity component is zero. Outlet pressure that was chosen as outlet boundary condition, fixed as air atmospheric pressure for all cases. Momentum, energy and mass transfers were investigated in steady state condition. The condensation rate was governed by the rate of diffusion of water vapour toward cold plates and controlled with source terms at the wall-adjacent cells. Cold wall was assumed as liquid-vapour interface and liquid film thickness was neglected. Both convective and diffusive components were taken into consideration for mass flux calculations at interface was assumed as impermeable to the air. 
The computational mesh was generated and checked for mesh-independent solution. Furthermore, the mesh near the plate surface and inlet boundary was refined in order to ensure an accurate resolution of the temperature, velocity and concentration gradients. In the model panel of FLUENT, energy and species transport models were activated. The pressure based solver was used to solve governing equations for the conservation of mass, momentum, species and energy equations. The convergence criteria were set 1 x 10-3 for continuity and x, y momentum, and 5 x 10-7 for the energy and H2O. Humid air was taken as air and water vapour mixture. Viscosity, thermal conductivity, specific heat and density of water vapour and air were taken as a function of temperature. These equations were entered into the material properties panel of the FLUENT. Thus, when solving the energy equation, the properties changed as the temperature changed in the flow channel.
The model was run under different relative humidity, velocity and temperature at inlet region and results were analysed to show the effects of these parameters on heat and mass transfer. The analyses based on relative humidity difference demonstrated that latent heat increases as relative humidity increases. However, sensible heat does not change considerably. It was observed that higher air inlet velocity decreases thermal and solutal boundary layer thicknesses. Temperature and species axial distributions were shown in figures. Various values of inlet temperature was considered while relative humidity and velocity was fixed. The simulation results showed that sensible and latent heat is directly proportional to air temperature. It was also observed that latent heat is more sensitive to temperature changes.
The problem was also solved with empirical correlations for flat surface valid under fully developed flow regime were used to calculate heat and mass transfer rates analytically. The flat plate region was divided into twenty sub-regions in flow direction and solved step by step to increase accuracy. Thermophysical properties were taken as a function of temperature which was also used at the numerical solution. The calculation algorithm was prepared on a MS Excel worksheet. Required parameters are air inlet and surface conditions. The unknown outlet conditions were calculated according to empirical correlations with taking thermophysical properties dynamically for each region.
Finally, the problem of water vapour condensation from humid air in horizontal channel has been analysed numerically and analytically. There are nine simulation cases which are set up with different input relative humidity, velocity and temperature. The effects of inlet humid air parameters were investigated. By comparing the numerical simulation with the solution based on empirical correlations, the discrepancies are in almost same range for all case. Deviations between both solutions are about 12% for latent heat and 16% for sensible heat. These discrepancies are due to the fact that empirical correlation does not take into consideration developing flow. It was observed that near the entrance region heat and mass transfer coefficients are significantly high in the numerical simulations. The results of this study are expected to benefit the design and optimization of thermal systems concerning water vapour condensation from humid air.