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DOI: http://dx.doi.org/10.20534/AJT-17-1.2-30-34
Maksudova Nasima Atkhamovna, Senior Lecturer, department: Strength of Materials, faculty: Mechanical Engineering Tashkent State Technical University Iskandarov Asilbek Akrom ugli, bachelor, department: Thermal Engineering, faculty: Energetics,
Tashkent State Technical University named after Abu Raykhon Beruni, Tashkent, Republic of Uzbekistan E-mail: asilbek.iskandarov17@gmail.com
Research project of metal oxide nanofluids reaching an increase of heat transfer rate capacity in solar absorption refrigerator
Abstract: Solid metallic materials, such as silver, copper and iron, and non-metallic materials, such as Alumina, CuO, SiC and carbon nanotubes, have much higher thermal conductivities than heat transfer fluids (HTFs). It is thus an innovative idea trying to enhance the thermal conductivity by adding solid particles into HTFs and can be used as heat transfer media in the solar absorption refrigeration system. AgO nanofluid with weights percent of 0.1, 0.2, 0.3 and 0.4 %, which compared in the ability of transfer and storage the heat with distilled water, it is found that the suitable weight percent was 0.1 wt %. The flow rate required supplying heat input to generator and the volume of hot fluid storage required to operate the refrigerator for 24 hours has been calculated. Experimental and theoretical results obtained from the present work show a good improvement by comparing with literatures.
Keywords: nanofluid, absorption refrigeration system, energy storage, heat transfer, heat capacity.
Generally, nanofluids are formed by dispersing large surface area to volume ratio, dimension-dependent nanometer-sized particles (1-100 nm) or droplets into physical properties, and lower kinetic energy, which can HTFs. Nanoparticles have unique properties, such as be exploited by the nanofluids. At the same time, the large
surface area make nanoparticles better and more stably dispersed in base fluids. Compared with micro-fluids or milli-fluids, nanofluids stay more stable, so nanofluids are promising for practical applications without causing problems mentioned above. Nanofluids well keep the flu-idic properties of the base fluids, behave like pure liquids and incur little penalty in pressure drop due to the fact that the dispersed phase (nanoparticles) are extremely tiny, which can be very stably suspended in fluids with or even without the help of surfactants [4].
Solar energy conversion to electricity is achieved primarily by using (a) photovoltaic technology, or (b) by harnessing solar thermal-energy. At larger scales of production, solar thermal techniques are more reliable and cost effective (as opposed to photovoltaic technologies), since these platforms can provide uninterrupted power supply in the off peak time (at night and during cloud cover). Solar thermal power plants rely on high temperature thermal storage units for continuous operation. Typical solar thermal-energy storage facilities require the storage medium to have high heat capacity and thermal conductivity. Contemporary commercial solar thermal units use energy storage facilities that operate at 400 °C and typically use mineral oil based storage medium as well as heat transfer fluids. It is estimated that pushing the storage facility to operate at 500-600 °C or higher can make the cost of solar power competitive with coal fired power plants in near future. However, few materials are compatible with the cost and performance requirements for such high-temperature thermal-energy storage. Typical materials used as HTF and for high-temperature thermal-energy storage include Na-K eutectics and alkali metal salt eutectics (e. g., NaNO3 , KNO3 , KCl, etc.). However, these materials have low thermo-physical properties. Hence, there is a need to find better performing thermal-energy storage technologies and materials that are cost effective. It should be noted that novel materials (such as nano material additives) can become cost effective if they can increase the operating range of the storage facilities to higher range of temperatures. For high temperature thermal-energy storage, compatible materials include molten salts and their eutectics, such as alkali-nitrate, alkali- carbonate, or alkali-chlorides. However, those molten salts have relatively low heat capacity — usually less than 0 J/(g °K) (in contrast to specific heat capacity of water which is 4.1 J/(g °K) at room temperature) [1].
Due to energy shortage in some regions, especially after the energy crisis of the 1970's, solar energy as
a renewable energy source has once again become a popular energy source. Research and development in the solar energy field has grown rapidly, along with research in solar cooling. With the invention of the DC-motor, photovoltaic (pv) technology was first used for pumping water. Later the pump motor was modified to drive the vapour compression system. PV-driven water pumps and refrigerators have since become a relatively large business. Subsequently, researchers have integrated so-called Peltier coolers with PV-panels to simple, yet inefficient solar coolers. These systems are used in the cold chain projects of the World Health Organization [2]. Contradictory reports in the literature demonstrate the degradation in specific heat of the fluids on doping with nanoparticles. Zhou and Ni [5] reported the reduction in specific heat of water by as much as 50 %, when doped with aluminum oxide nanoparticles, with progressive increase in volume fraction from 0 % to 21.7 %. The aim of this research is to enhance the heat transfer rate in the solar absorption refrigeration system by replacing liquid paraffin wax by AgO nanofluid.
Solar Driven Cooling System
Any solar cooling system design essentially consists of two parts: the cooling unit that uses thermal cycle is not different from those used in conventional refrigerators, and heat source with the solar flat plat collector or focus operation [5].
1. The cooling unit
The absorption diffusion refrigerator machine is designed according to the operating principles of the refrigeration machine mono pressure invented by Platen and Munter (Unique Gas Products Ltd).This machine used three operating fluids, water as the absorbent, the ammonia as refrigerant, and hydrogen as inert gas used in order to maintain the total pressure constant, which is composed of the principal following elements:
1.1. The boiler
A precise heat (electric heater element or gas flame) is applied to the boiler to begin operation. Heat is transferred from the outer shell of the boiler through the weak ammonia solution to the perk tube. The perk tube is provided with a rich ammonia solution (a high percentage of ammonia to water) from the absorber tank. When heated, the ammonia in the rich ammonia solution begins to vaporize (sooner than the water would) creating bubbles and a percolating effect. The ammonia vapor pushes the now weakening solution up and out of the perk tube. The ammonia vapor (gas) leaving the perk tube goes upward towards the top of the cooling unit, passing through the rectifier. The rectifier is just
a slightly cooler section of pipe that causes water that might have vaporized to condense and drop back down. The water separator at the top of the cooling unit (only on some models) prevents any water that might have escaped the rectifier to condense and fall back. After this point, pure ammonia vapor is delivered to the condenser. Meanwhile, back at the perk tube, the weaker solution expelled from the perk tube by the ammonia vapor drops into the weak ammonia solution surrounding the perk tube. Here, a little more ammonia vapor is generated and rises. The weak ammonia solution flows down ward and through the outer shell of the liquid heat exchanger, where heat is transferred to the rich ammonia solution on its way to the perk tube. The weak ammonia solution then flows to the top of the absorber coils and enters at a cooler temperature.
1.2. The condenser
Ammonia vapor enters the condenser where it is cooled by air passing through the metal fins of the condenser. The cooling effect of the condenser coupled with a series of step-downs in pipe size forces the ammonia vapor into a liquid state, where it enters the evaporator section.
1.3. The evaporator
Liquid ammonia enters the low temperature evaporator (refrigerator/freezer) and trickles down the pipe, wetting the walls. Hydrogen, supplied through the inner pipe of the evaporator, passes over the wet walls, causing the liquid ammonia to evaporate into the hydrogen atmosphere at an initial temperature of around -28.88 °C. The evaporation of the ammonia extracts heat from the refrigerator/freezer. At the beginning stages, the pressure of the hydrogen is around 24.5 kg/cm 2, while the pressure of the liquid ammonia is near 0.98 kg/cm 2. As the ammonia evaporates and excess liquids continues to trickle down the tube, its pressure and evaporation temperature rise. The liquid ammonia entering the high temperature evaporator (refrigerator portion) is around 3.08 kg/cm 2, while the pressure of the hydrogen has dropped to 22.75. Under these conditions, the evaporation temperature of the liquid ammonia is -9.44 °C. Heat is removed from the refrigerator box through the fins attached to the high temperature evaporator. The ammonia vapor created by the evaporation of the liquid ammonia mixes with the already present hydrogen vapor, making it heavier. Since the ammonia and hydrogen vapor mixture is heavier than the purer hydrogen, it drops down through the evaporators, through the return tube to the absorber tank.
1.4. The absorber
When the ammonia and hydrogen vapor mixture enters the absorber tank through the return tube, much of the ammonia vapor is absorbed into the surface of the rich ammonia solution, which occupies the lower half of the tank. Now lighter, the ammonia and hydrogen mixture (now with less ammonia) begins to rise up the absorber coils. The weak ammonia solution trickling down the absorber coils from the top (generated by the boiler) is "hungry" for the ammonia vapor rising up the absorber coils with the hydrogen. This weak ammonia solution eventually absorbs all the ammonia from the ammonia and hydrogen mixture as it rises, allowing pure hydrogen to rise up the inner pipe of the evaporator section and once again do its job of passing over the wetted walls of the evaporator. The absorption process in the absorber section generates heat, which is dissipated.
1.5. The Fuse
The fuse on many cooling units and in this graphic is a steel tube, the end of which is filled with solder. The plug is hollow and filled with solder. In either case, the fuse is the weak link of the system. If pressure inside the cooling unit were to rise beyond a reasonable level for some reason, the fuse is designed to blow and release the pressure. This would make the cooling unit inoperable, but is necessary for safety.
2. The solar Collectors
The major energy gains in the receiver in a solar collector are from the direct absorption of visible light from the sun and, additionally, the absorption of infrared radiation from the warm glass as shown in fig. 1. Important energy losses are infrared radiation emission, convective heat due to natural convection between the receiver and glass, as well as conduction of heat through the rear and sides of the collector. Therefore, the efficiency of the solar collector depends on all of these factors. The efficiency of the solar collector sub-system can be defined as the ratio of useful heat output to the total incident solar radiation (insolation) [2].
Results and Discussion
Results were obtained for using AgO nanofluid with different weights percent which are 0.1, 0.2, 0.3, and 0.4. Fig. 2 shows variation of temperature of fluid with time, from which we can see that the favorite weight percent that above the line of pure water and the suitable is 0.1 wt. %, which behave as high heat absorption to transfer it to the refrigeration system in the solar absorption refrigeration system.
Fig. 1. Energy Flows in a Single-Glazed Collector [8]
Fig. 2. Variation of Fluid Temperature with Time Heat Loss
From the T\Log diagram (Carl) the boiling point of rich ammonia-water mixture (33 %) is 130 °C and the weak mixture (12 %) temperature at 23 bar is about 190 °C. To operate the cooling unit with above condition, inlet generator temperature about 200 °C, and outlet temperature about 140 °C must be supplied from solar concentrator, then the flow rate of the working fluid required to achieve this operation is calculate as follow: In this study 2.52 hr. was measured to generate 1 kg. of ammonia with 649 kcal, and then the energy demand of the generator is:
= m • c • AT.
(2) (3)
^ = 257.5М/ = 2.52 /hr
= 257.5 • 1.166667 = 300 W.
Q=
(1)
p
AT = (T - T ).
V go g^
The specific heat (cp) ofAgO is 66J/(mole °K), then the mass flow rate of heat exchange fluid required (AgO + water) can be calculated based on the proposed entrance and exit temperatures of the oil in the generator [3]:
Cp of water = 4.2 J/(g oK). We select 0.1 wt. % ofAgO nanofluid.
Molecular weight of AgO = 232 g/gmole;
Cp mix = 0.284 • 0.1/100 + 4.2 = 4.2 J/(g oK);
Cp of AgO = 66/232 = 0.284J/(g oK) m = 300
4.2 • (200 -140)
1.19 g
The density of AgO is 7.5 g/cm 3 then: The density of water = 1 g/cm 3; pmix=7.5 • 0.1/100 + 1 • 0.9 = 0.907 g/cm 3. The volumetric flow rate is:
-119- = 1.3 cm 3/s = 4.68 liter/hr. (4)
0.907
The required hot fluid volume for 24 hr. is: 4.68 • 24 = 112.3 liter/day
Table 1. - Comparison Between Results of the Present Work and Obtained from Ref. (1)
Item Ref. [5] Present work Improvement Ratio ( %)
Mass Flow Rate (g/s) 2.28 1.19 46.49
Volumetric Flow Rate (liter/hr) 10.27 4.68 54.43
Hot Fluid Volume (liter/day) 246.5 112.3 54.44
Heat Capacity (J/(g°K)) 2.19 4.2 91.78
Conclusions
From the present work we can report the following conclusions:
1. Using AgO nanofluid increase the heat transfer rate.
2. Using AgO nanofluid reduce the required hot fluid volume if compared with [5] as shown in table 1.
3. Using AgO nanofluid reduce the volumetric flow rate if compared with [5] as shown in table 1.
4. Using AgO nanofluid increasing the heat capacity if compared with [5] as shown in table 1.
Nomenclature Cp — Specific heat capacity (J/(g oC)); C mix — Specific Heat Capacity of the Mixture (AgO and
water) (J/(g oC)); m — Generator mass flow rate (kg/s); Q — Generator heat input (W); Tgi — Generator inlet temperature (oC); Tgo — Generator outlet temperature (oC).
Greek Symbols pmix — Density of mixture (AgO and Water);
AT = (T - T ) — Temperature Difference of the gen-
g v g° g^ r °
erator.
References:
1. Donghyun Shin et al. Enhancement of specific heat capacity of high-temperature silicananofluids synthesized in alkali chloride salt eutectics for solar thermal-energy storage applications//International Journal of Heat and Mass Transfer. - 2011. - № 54. - P. 1064-1070.
2. Pridasawas W. Solar-Driven Refrigeration Systems with Focus on the Ejector Cycle. Doctoral Thesis submitted to Division ofApplied Thermodynamic and Refrigeration, Department of Energy Technology, School of industrial Engineering and Management, Royal Institute of Technology, KTH, 2006.
3. Narziev A. N., Iskandarov A. A. Improvement of autonomous system of the electrical supply, maintaining renewable power sources//International Scientific Review. - 2016. - № 3 (13). - P. 14-17.
4. Zenghu Han. Nanofluids with Enhanced Thermal Transport Properties. - Department of Mechanical Engineering University of Maryland at College Park College Park, Maryland, 2008.
5. Zhou, S. Q and Ni, R., , Measurement of the Specific Heat Capacity of Water-Based Al2O3 Nanofluid//Appl. Phys. Lett. - 2008. - № 92. - P. 93-123.
The properties of polyethylene nanocomposites based on organo-modified montmorillonite
DOI: http://dx.doi.org/10.20534/AJT-17-1.2-30-35-37
Turaev Erkin, Ph. D. Independent researcher Tashkent chemical-technological institute, The faculty of chemical technology of fuel and organic substances, Uzbekistan, Tashkent E-mail: turaev08@yahoo.com
Mikitaev Abdulahj
Doctor of Chemistry Professor of Kabardino-Balkarian State University named by H. M. BerbekovRussia, Nalchik
Djalilov Abdulakhatj Doctor of Chemistry, Professor, Director of Tashkent State Unitary Enterprise Research Institute, Uzbekistan, Tashkent
The properties of polyethylene nanocomposites based on organo-modified montmorillonite
Abstract: Research the possibility of obtaining nanocomposite materials by the process of melt-mixing using organo-modified montmorillonite. Studied the the effect of the organoclay on the physical and mechanical, thermal properties of high density polyethylene.
Keywords: Polyethylene; Organo-modified montmorillonite; nanocomposite; Mechanical properties; Thermal stability.
Introduction
In recent decades, the task of developing new materials is achieved by the modification of the base grades of industrial polymers. One way of adjusting the properties of polymer materials, is to obtain composite materials filled with nano size particles. It's due to the fact that such composite materials have a number of significant advantages. When incorporating nanoscale fillers in a polymer matrix, there is an increase of modulus, impact strength, thermal stability, chemical stability to solvents, flammability and decrease gas diffusion and permeability in polymers occurs.
In connection of above mentioned, the development and study of the properties of nanocomposites based on high density polyethylene (PE) and nanoscale particles is a very urgent task that allows to expand the scope of PE.
Methods of organic modification of montmorillonite
It is known that the main problem of creating layered silicate nanocomposites is the incompatibility of the organic (polymer) and inorganic (layered silicate) constituent of composites. This problem can be solved by using organo-modified layered silicate as an alternative. This product is the replacement of inorganic cations in the galleries of the layered silicates with organic cations, as shown in fig. 1.
As a nanoscale PE filler, we used montmorillonite (MMT), which is derived from bentonite clay deposits of Gerpegezh (Kabardino-Balkarian Republic).
Organic modifier that has been used for modification of organic MMT is shown in table 1.
Fig. 1. Scheme of organic modification of montmorillonite
