Recently, the heavy demand on electricity in summer season and environmental problems have made absorption heat pump cycle more attractive for both residential and industrial applications. In order to improve the efficiency of the absorption machine, the low heat and mass transfer in the absorber should be enhanced by the mechanical and physico-chemical ways. This study dealt with the phsyco-chemical method, the enhancement of the heat & mass transfer by surfactant. It is well known that a small amount of surface-active agent (surfactant) can induce the Marangoni convection which is caused by the surface tension gradient with respect to the temperature, absorbent concentration, and surfactant concentration. Many studies on the heat & mass transfer enhancement by surfactant in the literature are concentrated on the well-known working fluids, LiBr + water and water + ammonia. In our laboratory, several LiBr-based solutions were suggested as the potential working fluids for an absorption heat pump. For the application of the new working fluids, the heat and mass transfer characteristics in the absorber should be verified. The overall objective of this work is to supply the experimental information on the enhancement of the heat and mass transfer by surface-active additives and to investigate the characteristics of the heat and mass transfer by developing the proper design equation for the new working fluid-additives pairs. The new working fluids treated in this work are LiBr + 1,3-propanediol + water solution (LiBr/1,3-propanediol = 3.5 by mass) and LiBr + LiI + 1,3-propanediol + water solution (LiBr/LiI = 4 by mole and (LiBr+LiI)/1,3-propanediol = 4 by mass). The additives considered in this study were n-octanol and 2-ethyl-1-hexanol. To attack these subjects, measurements or estimations of the related transport and theromphysical properties were performed. The absorption experiments in the static pool type were performed and also carried out for the preliminary test on the new working fluid-additive pairs. The minisorber, experimental apparatus to investigate the characteristics of heat & mass transfer in the falling film absorption was designed and constructed. The water vapor absorptions in the falling film of new working fluids with and without additive were carried out.
For the measurement of thermal conductivity, an experimental apparatus was constructed using transient hot wire method. The ordinary transient hot wire method was modified for the electrolyte solution. Tantalum wire anodized to form tantalum pentoxide on its surface was prepared and used as a heater and temperature sensor. The heats of absorption for the data reduction in the falling film absorption experiments were determined from the heats of dilution. The heats of dilution for new working fluids were measured by Isoperibol Solution Calorimeter. It is known that the surface tension gradient with respect to the absorbent concentration and temperature are closely related to the onset of the heat & mass transfer enhancement. The surface tensions of the new working fluids with/without surfactant were measured by capillary rise method and ring method. For the analysis of the absorbent concentration, refractive indices were measured by Abbe refractometer. The addition of surfactants, n-octanol and 2-ethyl-1-hexanol, did not change the refractive index. Therefore the refractive index with absorbent concentration could be used as a standard calibration curve with additives. The water diffusivities in the working fluids were estimated based on the Stokes-Einstein equation. The diffusivities were used for the development of the mass transfer correlation and explanation of the absorption behavior.
In order to understand the heat & mass transfer characteristics for new working fluids, water vapor absorptions into static pool and falling film type absorber were carried out. The experimental conditions were set to have the same driving force for water vapor absorption. For both experiments, the absorption capacity was compared between the newly developed working fluids and LiBr + water solution. Without additive, LiBr + water solution showed better absorption rate in the static pool type absorber at the same vapor pressure while LiBr + 1,3-propandil + water solution was poorest. The high viscosity and low diffusivity of water for new working fluid seemed to be responsible for this behavior. With increasing the additive concentration up to 500 ppm, the absorption rate increased remarkably. LiBr + LiI + 1,3-propandil + water solution showed comparable absorption capacity. The maximum absorption was achieved near the solubility limit of the surfactant. The surfactant island was not the necessary condition for the enhancement. The effect of surface tension gradient with respect to the absorbent concentration was inhibited by the temperature effect on the surface tension. For the falling film absorption experiments, the experimental absorption data were presented in terms of the film Reynolds number. Without additive, the mass transfer rate increased until film Reynolds number increased to about 40 due to the wavy motion. But the film development acted as the resistance to prevent further increase in water vapor absorption. Mass transfer correlations for each working fluid were obtained using a simple power law. For all working fluids at Ref = 40, the mass transfer rate increased with additive concentration up to near the solubility limit of the surfactant. Similar to the experimental results from a static pool type absorber, the mass transfer enhancement occurred before solubility limit and maximized near the solubility limit. Judging from the absorption behavior and high solubility, LiBr + LiI + 1,3-propandil + water with 2-ethyl-1-hexanol solution can be a promising working fluid for an air-cooled absorption chiller.
For overall aspect, the absorption characteristics seemed to be related to the properties such as viscosity, diffusivity, and surface tension. More detailed study on the absorption enhancement for the various working fluid-additive pairs should be undertaken in the future in order to verify and understand the enhancement by additives.
최근 여름철 전력부하가 심해지고 냉방기에 사용되어지는 작동유체에 대한 환경적 규제가 심해지면서 흡수식열펌프 사이클의 가정용, 산업용으로의 적용에 많은 관심이 모아지고 있다. 흡수식기계의 낮은 성능을 향상 시키기 위해서는 흡수기에서의 열 및 물질전달을 기계적 혹은 물리화학적 방법으로 촉진시켜야한다. 본 연구에서는 계면활성제를 이용한 열 및 물질전달촉진, 즉 물리화학적인 방법에 대하여 다루고 있다. 흡수식열펌프에서 아주 작은 양의 계면활성제가 작동유체에 포함될 경우 냉매흡수시 표면장력구배에 의한 마랑고니대류가 발생되어지고 이로 인해 열 및 물질전달이 크게 향상되는 것으로 알려져 있다. 대부분의 연구는 잘 알려진 작동유체인 리튬브로마이드 수용액과 암모니아 수용액에 대해 제한되어 있는 실정이다. 새로운 작동유체가 실제기계에 적용되어지기 위해서는 열역학적인 물성뿐 아니라 열 및 물질전달특성이 파악되어야 한다. 본 연구에서는 새로이 제안되어진 리튬브로마이드기반 유체에 대해서 계면활성제의 유무에 따른 열 및 물질전달 촉진특성을 파악하고 새로운 작동유체-계면활성제계에 대해서 적절한 열 및 물질전달관계식을 도출하는 것을 목적으로 하였다. 본 연구를 통하여 다루어진 작동유체는 LiBr + 1,3-propanediol + water 용액과 (LiBr/1,3-propanediol = 3.5 by mass) LiBr + LiI + 1,3-propanediol + water (LiBr/LiI = 4 by mole and (LiBr+LiI)/1,3-propanediol = 4 by mass) 용액이고 첨가제로 사용된 계면활성제는 n-octanol과 2-ethyl-1-hexanol 이다. 본 연구목표의 달성을 위해 우선 열 및 물질전달에 관계되어지는 전달물성 및 물리화학적물성을 측정 또는 추산하고 정체형흡수기를 이용하여 새로운 작동유체-계면활성제에 대하여 예비테스트로 정체형 흡수실험을 수행하였으며 이 실험결과를 기반으로 유하액막흡수실험의 시스템과 실험조건을 결정하였다. 마지막으로 유하액막흡수실험을 수행하기 위해서 미니소버를 제작하였고 첨가제유무에 따른 증기흡수실험을 수행하였다.
열전도도의 측정을 위해서 비정상열선법을 기반으로 하는 실험장치를 제작하였고 전해질용액에 적용될수 있도록 탄탈룸선을 산화시켜서 절연을 수행하였다. 유하액막흡수실험 해석을 위해서 흡수열이 필요한데 이는 Isoperibol 용액열량계를 이용하여 측정된 희석열과 물의 엔탈피로부터 계산하였다. 새로운 작동유체의 표면장력은 계면활성제가 포함되지 않은 경우에는 모세관오름방법으로 계면활성제가 포함된 경우에는 링방법을 이용하여 측정하였다. 흡수실험후 농도분석을 위하여 굴절률을 측정하였고 실험결과 계면활성제가 포함된 경우에도 농도분석에 사용되어질수 있음을 확인하였다. 작동유체내에서 물의 확산계수는 스톡스-아인쉬타인방법을 기반으로 하여 추산하여 열 및 물질전달경향해석에 사용하였다.
새로운 작동유체의 열 및 물질전달특성을 파악하기 위해 정체형 및 유하액막흡수기에서 증기흡수실험을 수행하였다. 흡수제의 농도는 각각의 리튬브로마이드용액과 같은 증기압을 갖도록 조정하였다. 계면활성제가 포함되지 않은 경우에 리튬브로마이드용액이 가장 큰 증기흡수율을 보였고 LiBr + 1,3-propanediol 용액이 가장 낮은 흡수율을 보였는데 이는 새로운 용액의 높은점도 및 낮은 확산계수에 의한 것으로 보여진다. 첨가제농도를 약 500 ppm 까지 증가시킴에 따라 흡수율은 급격하게 증가하였고 LiBr + LiI + 1,3-propanediol 용액은 리튬브로마이드용액보다 조금 낮은 값을 나타냈다. 최대흡수촉진은 첨가된 계면활성제의 최대 용해도근처에서 나타났다. Salting out 모델에 근거할 때, 새로운 작동유체에서는 표면장력에 대한 온도의 영향이 농도에 대한 영향보다 더 크게 나타나서 계면활성제가 첨가되지 않은 경우에도 리튬브로마이드용액과 같이 표면난류는 발생하지 않았다. 유하액막 흡수실험에서는 실험데이타를 필름레이놀즈넘버로 표현하였다. 첨가제가 없는 경우 물질전달율은 필름레이놀즈넘버가 약 40 정도일때 까지 wavy 모션에 의해 증가되었으나 그이상에서는 두꺼워진 액막필름이 저항으로 작용하여 더 이상 크게 증가하지 않았다. 실험데이타로부터 너셀넘버와 셔우드넘버를 필름레이놀즈넘버에 대한 간단한 함수로 표현하여 열 및 물질전달관계식을 표현하였다. 필름레이놀즈 넘버를 40으로 고정하고 계면활성제로 2-ethyl-1-hexanol 을 약 1000 ppm 까지 첨가함에 물질전달율은 크게 증가하였고 계면활성제의 최대용해도근처에서 최대값을 나타내었다. 흡수실험결과와 작동영역으로 판단할 때 LiBr + LiI + 1,3-propanediol 용액과 2-ethyl-1-hexanol 계의 경우가 유망한 새로운 작동유체가 될수 있을 것으로 판단된다.