Gözenekli ortamda darbeli hava akışı durumunda ısı geçişinin deneysel incelenmesi
Experimental investigation of heat transfer in pulsating air flow with a porous medium
Yazar: ALİ MURAT BİNARK
Danışman: PROF. DR. MUSTAFA ÖZDEMİR
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Isı-Akışkan Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Bilim ve Teknoloji = Science and Technology ; Enerji = Energy ; Makine Mühendisliği = Mechanical Engineering

Özet:
Gelişen teknolojinin getirdiği hızlı ısınma sorunu, hızlı ve verimli soğutma teknolojilerine olan ihtiyacı arttırmakta, kanal içi akışlarda ısı geçişini arttırmanın önemini göstermektedir. Kanal içine yerleştirilen gözenekli ortam ve sabit debideki akışkana bir titreşim kazandırmak ısı geçişini arttırmak amacıyla üzerinde çalışılan alanlardan ikisidir. Bu iki teknolojinin birleştirilmesiyle elde edilebilecek yüksek ısı geçişi oranı gözenekli ortamda titreşimli akışlarla ilgili çalışmaların sayısını arttırmıştır. İki yönlü git-gel akışının aksine tek yönlü darbeli akış için literatürdeki çalışma sayısının azlığından dolayı bu çalışmada metal köpük içinde darbeli hava akışı durumunda ısı geçişinin deneysel olarak incelenmesi amaçlanmıştır.
Bu çalışmada içinde metal köpük bulunan test odasının yüzeyine yerleştirilen ısıtıcı ile sabit ısı akısı uygulanırken, kanaldan geçen havaya, zamana bağlı olarak dönen bir valf aracılığıyla titreşim kazandırılmış ve oluşan darbeli hava akışının metal köpüklü test odasında ısı çekme karakteristiği incelenmiştir. Bunun için test odasının yüzeyine girişten çıkışa kadar aralıklarla yerleştirilen termoelemanlarla sıcaklık ölçümü alınmıştır. Akışkanın zamana bağlı değişen debisi ise orifis ile ölçülmüştür.
Bu çalışmada ayrıca gözenekli ortamdaki darbeli hava akışının hidrodinamik özellikleri de incelenmiştir. Bunun için test bölgesinin girişi ve çıkışı arasındaki basınç farkı bir basınç transdüseri ile ölçülmüştür.
Toplam 21 adet deney yapılmıştır. Bunlardan bir kısmı sabit debide, bir kısmı ise değişken debide, bir kısmı sabit bir ısı akısı altında bir kısmı ise ısı akısı uygulanmadan yapılmıştır. Isıtıcı gücünü, fan devir sayısını ve motorlu darbe valfinin dönme frekansını değiştirmek için sistemdeki üç adet varyakla üç parametre üzerinde değişiklik yapılarak deneyler tamamlanmıştır. Üç farklı fan gücünde, 4.5-6.4 g/s'lik ortalama kütlesel debilerde, çalışılmıştır. Bu çalışma aralığı 5932-8825 aralığındaki Reynolds sayılarına karşılık gelmektedir. Deneylerin 17 tanesinde 106 W değerinde sabit bir ısı akısı uygulanmıştır. Deneylerin 2 tanesi daimi akış deneyi, 19 tanesi ise darbeli akış deneyidir. Darbeli akış deneyleri 0.65-3.30 Hz debi frekansı aralığında gerçekleştirilmiştir. Elde edilen verilerle sürtünme katsayısı ve Nusselt sayısı değerleri hesaplanmıştır.
Darbeli akış durumunda elde edilen sürtünme katsayısı değerlerinin aynı ortalama Reynolds sayısındaki daimi akıştaki sürtünme katsayısından yüksek olduğu ve daimi akıştaki sürtünme katsayısına oranının Reynolds sayısından bağımsız olarak debi frekansı yükseldikçe arttığı sonucuna ulaşılmıştır.
Isı geçişi deneylerinde sıcaklıkların darbe etkisinden etkilenmediği, zamandan bağımsız olduğu görülmüştür. Nusselt sayısında darbeli akış durumunda anlamlı bir değişim gözlenememiştir.

Abstract:
The problem of rapid heating caused by the developing technology increases the need for fast and efficient cooling technologies and shows the importance of increasing the heat transfer in the channel flow. The porous medium placed in the duct and to give a vibration to the stedy fluid flow are two of the studied areas in order to increase the heat transfer. The high heat transfer rate, which can be achieved by combining these two technologies, has increased the number of studies on vibrating flows in porous media. In porous media, a fluid mixing effect occurs and vortexes significantly increase the heat transfer. In vibratory flow, the non-slip condition on the wall causes different vibration amplitudes to occur between the mass of the fluid and the boundary region. Thus, depending on the amplitude and frequency of the flow vibration, the contact between the hot portion and the cold portion of the fluid increases. This leads to an increase in heat transfer. The vibrating flow is divided into two as oscillating flow and pulsating flow. The oscillating flow is the state in which the fluid swings through the system by means of a piston cylinder. The flow oscillates around zero. The pulsating flow is the state where the fluid oscillates around a non-zero mean flow rate without changing direction with the aid of an pulsating element. In contrast to the oscillating flow, the number of experimental studies in the literature is not sufficient for pulsating flow. Current studies have focused on numerical solutions. Therefore, in this study, in the duct with metal foam, under the pulsed flow formed by a pulse-actuated valve, experiments were carried out on aluminum foam having 10 PPI pore density at different pulse frequencies, different average mass flow rates, with constant heat flux provided by a heater. It is aimed to observe the effect on flow and heat transfer characteristics.
Test installation consist of an orifice and pressure transducer for measuring flow rate, a test zone with a porous medium, another pressure transducer for measuring the pressure drop in this test zone, a heater surrounding the test zone, and insulation, thermocouples on the surface of the test zone, at the inlet and outlet of the test zone and on the insullaton outer surface, a rotating valve that creates the pulse effect in the flow at the outlet of the test zone, and a suction fan which draws air out of the system. The air enters the system properly with a nozzle. The air passing through the wire mesh straightener reaches the orifice evenly. The air exiting the orifice enters the test zone after passing through a long duct. There is an aluminum foam material in the test zone whose diameter is the same as the diameter of the channel. Aluminum foam is soldered to the aluminum pipe surface in which it is located. Around the test zone there is a cylindrical heater which applies a constant heat flux. The area around the heater is covered with glass wool insulation material. At the outlet of the test zone is a valve which pulses the flow. A disc connected with the valve rotating at the same angular speed as the valve rotates outside the channel. The rotational speed of this disc is measured by tachometer and the rotational frequency of the valve is determined. Then the air passing through the suction fan is transferred to the outdoor environment.
In the installation, heater, motorized valve and suction fan are supplied with variants. Thus, the power of the heater, the speed of the valve and the fan can be changed. In addition, the power of the heater is determined by measuring the electrical resistance and instantaneous voltage values.
In the test zone, in an aluminum channel of 63.5 mm outer diameter and 50.6 mm inner diameter; 322 mm long, 10 PPI pore density, 88.5% porosity, ERG brand open cell aluminum foam.
A total of 21 experiments were conducted. Some of these are made at steady flow, some at pulsated flow, some under a constant heat flux and some without heat flux. In order to change the heater power, fan speed and the rotation frequency of the motorized pulse valve, the experiment was completed by making changes on three parameters with three variants in the system.
In A1-A4 experiments, no heat flux was applied and in B0-B6, C1-C4 and D0-D6 experiments, a heat flux of 106 W was applied.
Experiments were performed in each experimental group by starting from low valve frequency and increasing the frequency. In the group B and D, the steady flow tests are performed in the position of the valve to give the same average mass flow as in the case the valve rotates, and these are called B0 and D0. Apart from these, A1-A4, B1-B5, C1-C4, D1-D6 tests are pulsating flow tests at increasing valve frequencies.
Three different fan power, average mass flow rates of 4.5-6.4 g/s were studied. This study corresponds to Reynolds numbers in the range of 5932-8825.
Two of the experiments were steady flow test and 19 were pulsating flow test. Pulsating flow experiments were carried out in the 0.65-3.30 Hz flow frequency range.
In the pressure drop tests, it was observed that the frequency of the pressure drop differs from the flow frequency, secondary frequencies are activated and the effect of these secondary frequencies increases as the frequency increases. It has been observed that the mass flow amplitudes decrease with increasing frequency and gradually approach to the measurement error range. Pressure drop amplitudes were also investigated. It has been observed that the pressure drop amplitudes decrease to a certain frequency as the frequency increases, and the amplitudes decrease to the measurement error range at this frequency called "turning point frequency, and then tend to increase again as the frequency increases. In addition to this, frequency dependent change of pressure drop per unit length in different average mass flow conditions was investigated. The time average pressure drop tended to increase with a slight slope. On the other hand, the average mass flow was the main parameter affecting the pressure drop.
The results of the pressure drop test were then interpreted on the friction factor. Variation of friction factor depending on Reynolds number and flow frequency was investigated. For the same Reynolds number, but for different frequency experiments, the friction factor increased with increasing frequency. For the same flow frequency but different Reynolds number, it was observed that the friction factor decreased with increasing Reynolds number.
Then, in order to compare the pressure drop test results obtained for pulsed flow with the steady flow, the expression of "ratio of friction factor "is defined.
ηf is defined as the ratio of the friction factor found in the pulsating flow test to the friction factor obtained in the steady flow test for the same average Reynolds number. This defined ratio is equal to unity for the steady flow. As for the pulsating flow, it is seen that the effect of the pulsed flow on the pressure drop characteristic according to the situation in the steady flow, if it is greater than or less than one. It was concluded that the friction factor values obtained in the case of pulsating flow were higher than the friction factor in the steady flow of the same average Reynolds number and the ratio of friction factor increased as the flow rate increased independently of the Reynolds number.
In the heat transfer experiments, it was observed that the temperatures were not affected by the pulsating effect and were independent of time. Therefore, no frequency and amplitude can be mentioned at temperatures. On the other hand, it was observed that surface temperatures and air outlet temperature decreased as the average mass flow increased.
The mean surface temperature was obtained by taking the position average of the time average surface temperatures. The mean fluid temperature was obtained by taking the average of the time averages of the air inlet and outlet temperatures. In addition, the total heat is calculated over the voltage and electrical resistance of the heater. The heat lost to the environment was calculated by using the temperature of the outer surface of the insulation and the ambient temperature. The net heat transferred to the fluid was calculated by subtracting the heat lost from the total heat.
Local heat transfer coefficients were calculated in a certain part of the test area. The air temperature was measured at two points, the inlet of the test zone and the outlet of the test zone. However, surface temperatures were measured at thirty different points. The local heat transfer coefficient is based on the points where surface temperatures are measured. A trend line was determined between two points where the air temperature was measured and the air temperatures at the points where the surface temperatures were measured were estimated and the local heat transfer coefficients at these points were calculated. Since the temperature data in the inlet region and outlet region have a different slope depending on the location, they are not taken into account in the calculation of local heat transfer coefficient. Local heat transfer coefficients were found to increase with increasing mass flow, but were not affected by frequency.
In addition, fully developed region heat transfer coefficients and average heat transfer coefficients were calculated for each experiment. It is seen that heat transfer coefficients decrease with a slight slope as frequency increases in high mass flow rate. On the other hand, the average mass flow rate is the main parameter affecting the heat transfer coefficient.
The heat transfer test results were then interpreted on the Nusselt number. The change of Nusselt number due to Reynolds number and flow frequency was investigated. At the same flow frequency, it was observed that the Nuseelt number decreases as the Reynolds number increases for experiments with different Reynolds numbers. On the other hand, for experiments with the same Reynolds number but different frequency, no significant change was observed in the Nusselt number.
Then, in order to compare the heat transfer test results obtained for pulsating flow with the steady flow, the expression of "Nusselt number ratio" is defined.
ηNu is defined as the ratio of the Nusselt number found in the pulsating flow test to the Nusselt number obtained in the steady flow test for the same average Reynolds number. This defined ratio is equal to unity for the steady flow. As for the pulsating flow, it is seen that the effect of the pulsating flow on the heat transfer characteristic according to the situation in the steady flow depending on whether it is greater than or less than one. In the high Reynolds number, this ratio goes below one and tends to decrease. However, the maximum value of this change was calculated as 3.08%, which was lower than the uncertainty rate in the Nusselt number, and it was concluded that there was no significant change.