• Journal of Biosystems Engineering
• /
• v.29 no.2
• /
• pp.141-150
• /
• 2004

우리가 소비하는 가공 식품은 위생상 안전하도록 살균처리가 이루어진다. 식품 내에 존재할 수 있는 유해 세균은 일정 살균온도에서 살균에 필요한 시간 동안 노출되면 사멸하며, 일반적으로 살균온도가 높을수록 살균에 필요한 시간은 단축된다. 연속살균장치는 혼합 및 저장탱크에 담겨진 식품을 점프로 이동시키면서 가열 열교환기에서 살균온도로 가열하고 단열관을 거치는 동안 살균온도를 유지시켜 살균을 완료한다. 또한 살균된 식품은 냉각용 열교환기에서 상온으로 냉각되며 이 과정에서 회수되는 열은 저장탱크에서 유입되는 식품의 예열에 사용되어 에너지 효율을 제고하는데 사용되기도 한다. 이와 같이 관을 이동하면서 가열되는 살균장치는 기존의 배치식 살균방법에 비하여 균일하게 가열이 이루어지므로 130C의 고온으로 살균할 수 있어서 살균에 필요한 시간을 수초에서 수십초 정도로 단축시킬 수가 있고 그에 따라 열손상을 크게 줄일 수 있다. 또한, 상온으로 냉각된 식품을 포장함으로써 저렴한 가격의 포장용기를 사용할 수 있고 상온에서 저장할 수 있으므로 저장비용이 저렴한 장점이 있다. 그러나, 가공식품에 고기나 야채와 같은 고체 상태의 식품이 함유된 경우에는 액상 식품이 열 교환기에서 순간 가열되며, 고상 식품은 액상식품과의 대류에 의한 열전달로 가열된다. 이 과정에서 고상식품은 이동관 내벽이나 다른 고상식품과 부딪치거나 회전하면서 이동관 내부에서 자유롭게 운동하게 된다. 이 과정에서 액상식품과의 상대이동 속도가 발생하여 이것이 대류열전달에 영향을 미치게 된다. 이 상대이동속도에 따른 대류 열전달계수는 고상식품의 내부온도 결정에 사용되는 연속살균장치의 중요한 설계인자이다. 대류열전달계수는 연속살균장치에서 자유로이 이동하는 고상식품의 중심부의 온도를 측정하여 결정할 수 있으나 이는 현실적으로 어렵다. 따라서 본 연구에서는 고정된 고상식품에 액상식품을 이동시켜 상대속도를 재현하고 액상식품의 온도와 고상식품의 중심온도를 측정하는 장치를 개발하였으며, 각 상대속도와 액상식품의 점도 별 대류열전달계수를 결정하는 프로그램을 유한차분법을 이용하여 개발하였다. 이 장치를 분당 15, 30, 40 리터의 유량에서 유체의 점도를 0에서 15 centipoise 사이의 세 수준에서 정육면체 소고기를 모델 고상식품으로 내부 온도분포를 측정하였으며, 유한차분법 프로그램으로 대류열전달계수를 결정하였다. 대류열전달계수는 792에서 2,107 W/m$^2$로 분석되었다. 대류열전달 계수는 액상식품과의 상대속도가 증가함에 따라서 증가하였고, 점도가 증가함에 따라서는 감소하였다.

• Journal of the Korea Concrete Institute
• /
• v.15 no.2
• /
• pp.314-322
• /
• 2003

Pipe cooling method is widely used for reduction of hydration heat and control of cracking in mass concrete structures. However, in order to effectively apply pipe cooling systems to concrete structures, the coefficient of flow convection relating the thermal transfer between inner stream of pipe and concrete must be estimated. In this study, a device measuring the coefficient of flow convection was developed. Since a variation of thermal distribution caused by pipe cooling has a direct effect on internal forced flows, the developed testing device is based on the internal forced flow concept. Influencing factors on the coefficient of flow convection are mainly flow velocity, pipe diameter and thickness, and pipe material. Using experimental results from the developed device, the coefficient of flow convection was calculated. Finally, a general prediction model was proposed by theoretical procedures. The proposed prediction model is able to estimate the coefficient of flow convection with flow velocity and material properties of pipe. From comparison with experimental results, the coefficient of flow convection by this model was well agreed with those by experimental results.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.37 no.8
• /
• pp.725-732
• /
• 2013

This study examined the convective heat transfer characteristics of nanofluids flowing over a heated fine wire. Convective heat transfer coefficients were measured for four different nano-engine-oil samples under three different temperature boundary conditions, i.e., both or either variation of wire and fluid temperature and constant film temperature. Experimental investigations that the increase in the convective heat transfer coefficients of nanofluids in the internal pipe flow often exceeded the increase in thermal conductivity were recently published; however, the current study did not confirm these results. Analyzing the behavior of the convective heat transfer coefficient under various temperature conditions was a useful tool to explain the relation between the thermal conductivity and the boundary layer thickness of nanofluids.

• Journal of Korean Tunnelling and Underground Space Association
• /
• v.12 no.3
• /
• pp.223-237
• /
• 2010

The initial heating rate is well known as one of the most influencing factors on the occurrence of spalling and the loss of strength in concrete after fire initiation. In this study, a series of fire tests were carried out at different initial heating rates to find out its effects on the deterioration of tunnel structural members. Heat transfer analyses combined with an element elimination model were also carried out to verify its applicability in the same conditions as the fire tests. Moreover, the convection heat transfer coefficients compatible with fire test results were derived from parametric studies. In this course, their time-dependent variations were also analyzed at different initial heating rates. Finally, a numerical formula to estimate the heat transfer coefficients at the various initial heating rates was proposed by the interpolation of the results of numerical analyses.

• Journal of Bio-Environment Control
• /
• v.22 no.2
• /
• pp.108-115
• /
• 2013

This study was conducted to suggest a model to calculate the overall heat transfer coefficient of single layer covering for various greenhouse conditions. There was a strong correlation between cover surface temperature and inside air temperature of greenhouse. The equations to calculate the convective and radiative heat transfer coefficients proposed by Kittas were best fitted for calculation of the overall heat transfer coefficient. Because the coefficient of linear regression between the calculated and measured cover surface temperature was founded to 0.98, the slope of the straight line is 1.009 and the intercept is 0.001, the calculation model of overall heat transfer coefficient proposed by this study is acceptable. The convective heat transfer between the inner cover surface and the inside air was greater than the radiative heat transfer, and the difference increased as the wind speed rose. The convective heat transfer between the outer cover surface and the outside air was less than the radiative heat transfer for the low wind speed, but greater than for the high wind speed. The outer cover convective heat flux increased proportion to the inner cover convective heat flux linearly. The overall heat transfer coefficient increased but the cover surface temperature decreased as the wind speed increased, and the regression function was founded to be logarithmic and power function, respectively.

• Journal of Biosystems Engineering
• /
• v.11 no.1
• /
• pp.37-51
• /
• 1986

실린더로 부터 Prandtl수가 0.7인 주변공기로 전달되는 혼합대류 열전달현상이 Stream-Vorticitv 함수로 표시된 지배방정식으로 부터 유한차분법에 의해 분석되어졌다. Reynolds수와 Grashof수의 함수로서 혼합대류 열전달에서 Nusselt수의 값들이 조사되어 졌으며 이들로부터 순수자연대류와 혼합대류, 혼합대류와 순수강제대류사이의 범위를 정하였다. 그 외에도 본 연구에서는 실린더주변의 온도분포와, 공기속도분포, 속도분리점(Separation point), 마찰계수, 압력분포등이 조사되어졌으나 제 1편에서는 Nusselt수와 온도분포만이 연구결과로 주어 졌으며, 본시험의 이론적 결과치들은 발표된 실험치들과 잘 일치하였다.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.40 no.2
• /
• pp.103-110
• /
• 2016

Fine wires made from platinum have been used as sensors to evaluate the convection performance of nanofluids. However, the wire sensor is difficult to handle due to its fragility. Additionally, an unrealistic convective heat transfer coefficient (h) is obtained if a rigorous calibration process combined with precision equipment is not used for measurement. This paper proposes a new evaluation apparatus for h of nanofluids that uses a thermistor sensor instead of the platinum wire. The working principles are also explained in detail. Validation experiments for pure engine oil comparing h from the two sensors confirmed numerous practical benefits of the thermistor. The proposed system can be used as a useful tool to justify the adoption of developed nanofluids.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.36 no.1
• /
• pp.9-16
• /
• 2012

This paper describes a measuring apparatus that can be used to appraise the effectiveness of nanofluids as new heat-transfer-enhancing fluids. A couple of apparatuses using fine hot wires as sensors have been proposed for this purpose; however, they have a technical weakness related to the uncertain working conditions of the sensor. The present method uses the convective heat transfer coefficient from a hot wire as an indication of the heat transfer effectiveness of the nanofluid, where the temperature of the wire remains constant during the experiment. The operating principle and experimental procedure are explained in detail, and the validity of the system is tested with pure base fluids. The effects of particle concentration, velocity, and temperature on the heat transfer coefficients of the nanofluids are discussed comprehensively using the experimental data for graphite nanolubrication oil.

• Journal of the Korean Institute of Gas
• /
• v.27 no.3
• /
• pp.123-133
• /
• 2023

Commercial hydrogen fuel cell vehicles are charged by compressing gaseous hydrogen to high pressure and storing it in a storage tank in the vehicle. This process causes the temperature of the gas to rise, to ensure the safety to storage tanks, the temperature is limited. Therefore, a heat transfer model is needed to explain this temperature rise. The heat transfer model includes the convective heat transfer phenomenon, and accurate estimation is required. In this study, the convective heat transfer coefficient in the hydrogen fueling process was calculated and compared using various correlation equations considering physical phenomena. The hydrogen fueling process was classified into the fueling line from the dispenser to the tank inlet and the storage tank in the vehicle, and the convective heat transfer coefficients were estimated according to process parameters such as mass flow rate, diameter, temperature and pressure. As a result, in the case of the inside of the filling line, the convective heat transfer coefficient was about 1000 times larger than that of the inside of the storage tank, and in the case of the outside of the filling line, the convective heat transfer coefficient was about 3 times larger than that of the outside of the storage tank. Finally, as a result of a comprehensive analysis of convective heat transfer coefficients in each process, it was found that outside the storage tank was lowest in the entire hydrogen fueling process, thus dominated the heat transfer phenomenon.

• Proceedings of the Korean Society for Agricultural Machinery Conference
• /
• 1999.12a
• /
• pp.523-530
• /
• 1999

연속살균장치는 $130^{\circ}C$에서 $140^{\circ}C$의 초고온에서 연속적으로 식품을 열처리 하는 공정으로 재래 배치식 공정에 비하여 순간적인 짧은 시간이 소요되는 경제적인 공정이나, 액상과 고상으로 구성된 저산도 식품은 고상입자의 대류열전달 계수와 장치내 체류시간이 정확히 구명되지 않아서 연속살균기술이 성공적으로 적용되지 못하고 있다. 본 연구에서 연속살균장치에서의 액상식품과 고상식품사이의 대류열전달 계수를 예측하기 위하여 연속살균장치의 Hold tube에서 정육면체 모델 식품내부의 온도를 측정할 수 있는 장치를 개발하였다. 연속살균장치의 홀드튜브에서 정육면체 모델 식품의 온도변화를 예측할 수 있는 유한차분법을 이용한 시뮬레이션 모델을 개발하고 소고기를 대상으로 이 시뮬레이션 모델의 입력변수인 비열, 열전도도를 실험적으로 측정하여 사용하였다. 0.0에서 15.0 centipoise의 점도를 가지는 모델 액상식품의 15.6에서 45.2liter/min 의 유속에 대하여 액상과 소고기 정육면체의 대류열전달계수는 792에서 2107W/$m^2$K으로 예측되었다.