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A newborn incubator with a laminar flow unit.

INTRODUCTION

World Health Organization (WHO) defines the premature baby as the baby born in the 22nd-37th week of the pregnancy (Rojczyk and Szczygiel 2011). A baby, born before the 28th week of the pregnancy, can live inside of a newborn incubator with a thermoregulation system. Newborns can be classified as their weights into four categories. These categories vary between 800 and 2500 grams. Thermoregulation is obtained as these weights. New-born baby mortality rate is very high, especially in developing countries. Figure 1 shows the infant mortality rate (CIA 2015). Infant mortality rate is the number of deaths of infants under one-year-old per 1,000 live births. It is generally accepted as an indicator of the level of health in a country. In Turkey in 1950-1955 periods, the new-born infant mortality rate was around 217.77 while in 2010-2015 it was reduced by up to 12.77. Figure 1 shows that this rate was fewer than 5 in Canada, Europe, Japan, and Australia. In EU, this was found to be 4. The highest new-born infant mortality rates occur in underdeveloped countries. The new-born infant deaths, particularly the highest in African countries, were discussed by (Hippolite 2012) according to the status of the incubators, operational procedures, and control systems. New-born deaths occur due to disease-induced water loss. The decrease and increase in body temperature cause hypothermia and hyperthermia, respectively those results in death. For the survival of the babies, born before the expected date, a suitable environment has to be provided that close to the womb conditions and the condition inside the environment has to be adjusted. The parameters to be controlled are temperature, humidity, sound, and [O.sub.2] level.

Especially premature new-born babies, born before the 32th week, if the body weight is below 1500 grams the thermoregulation are not like normal babies. 85% of the heat exchange is provided through the placenta in the fetus and 15% of the heat exchange is provided via skin. After the birth, these babies whose skin is immature can be affected even 1[degrees]C temperature difference. Therefore, thermoregulation has a great importance for the premature newborn babies and should complete their growth in an incubator according to the body temperature and birth week.

There are different types of incubators, available in the market. The most common incubator type separates the interior of the incubator by a plexiglas material. The temperature, humidity, and [O.sub.2] level have to be controlled in this closed environment. A photo of the closed type incubator is shown in Figure 2(a). The plexiglas cover prevents the ingress of the bacteria into the incubator. However, (Hippolite 2012) showed that in the case of aperiodic cleaning of the incubators increase the possibility of the bacterial growth. In the case of an emergency, removal of the cover exposes the baby a large temperature difference. The incubators with radiant heaters, shown in Figure 2(b), regulate the temperature difference but cause moisture loss from the skin of the new-born baby that is critical for the survival.

(ASHRAE 1989) defines the body heat balance as follows:

M - W = [Q.sub.sk] + [Q.sub.res] = (C + R + [E.sub.rsw] + [E.sub.dif]) + ([C.sub.res] + [E.sub.res]) (1)

In this equation, M is the metabolic energy production, W is the mechanical work, [Q.sub.sk] and [Q.sub.res] are the heat loss from skin and the heat loss by inhalation, respectively. The term, C, in the right hand side of the equation shows the heat loss by convection from skin and R is the heat loss via radiation. [E.sub.rsw] and [E.sub.dif] are the heat losses by the evaporation from skin and heat loss from skin by moisture diffusion. [C.sub.res] and [E.sub.res] are convective and evaporative heat losses by inhalation.

(Wongkamhang et al. 2012) obtained temperature and velocity distribution inside the closed type incubator by utilizing COMSOL CFD commercial code. They investigated heating of the incubator with an air velocity of 0.5 m/s after 1800 second. The air introduced into the incubator from the head side of the baby and exhausted from the foot side of the baby. (Rojczyk and Szczygiel 2013) obtained temperature and velocity distribution for an incubator with radiant heater. The velocity distribution on the baby was found to be 0.1 m/s. (Fic et al. 2010) obtained CFD results of an incubator with a radiant heater. In the analysis, heat input for radiant heater was given as 600 W. Also, the body temperature of the baby and convective heat loss were given as 37[degrees]C and 0 W, respectively. Heat loss from the baby was calculated for various cases. (Ginalski et al. 2007) investigated temperature distribution for 32, 34, and 36[degrees]C inlet air temperatures in a closed type incubator and compared the results with experimental study. (Perez et al. 2013) suggested a laminar flow type incubator. The laminar flow type incubator aimed to solve excessive moisture loss problem in incubators with radiant heaters, access and temperature variation problems when the cover is opened for the closed type incubators.

In this study, experimental and numerical investigation of a laminar flow type incubator is performed. In the experimental study, a neonatal test simulator was used in order to obtain velocity, temperature, humidity, and sound results. This device is also used to calibrate the incubators in the hospitals. Initially, the closed type incubator experiments were performed. For the next step, laminar flow type incubator was manufactured and tested. In the numerical part of the study, closed type incubator was modeled. The proposed laminar flow unit type incubator was also modeled and the results were compared.

NUMERICAL STUDIES

In the numerical studies, a closed type incubator was modeled. In order to determine the difference between closed and laminar flow type, a solid model of the laminar flow type incubator was modeled. ANSYS Fluent 15 was used in the numerical analysis. Continuity, momentum, and energy equations were used and the equations were given below.

[partial derivative][rho]/[partial derivative]t + [partial derivative]/[partial derivative][x.sub.i] [[rho][u.sub.j] = 0 (2)

[partial derivative]/[partial derivative]t ([rho][u.sub.i]) + [partial derivative]/[partial derivative][x.sub.j] [[rho][u.sub.i][u.sub.j] + p[[delta].sub.ij] - [[tau].sub.ij]] = 0 i = 1, 2, 3 (3)

[partial derivative]/[partial derivative]t ([rho][e.sub.0]) + [partial derivative]/[partial derivative][x.sub.j] [[rho][u.sub.j][e.sub.0] + [u.sub.j]p + [q.sub.j] - [u.sub.i][[tau].sub.ij]] = 0 (4)

For the accuracy of the solution, the selection of the turbulence model has a great importance. In the analyses, the flow is accepted as laminar. In addition, k-" Low Reynolds Corrections Model was used in the simulations. Air is fed into the closed type incubator with the values given in Table 1. The body temperature of the neonatal is accepted as 36.5[degrees]C. According to the mesh independency results total mesh of the model was found to be approximately 834,500.

The boundary conditions of the laminar flow type incubator are given in Table 2. The body temperature of the baby is accepted same as closed type incubator. The optimum mesh for laminar flow type incubator was found to be 594,295.

RESULTS

The velocity and temperature contours of the closed type incubator were given in Figures 3(a) and 3(b). The velocity on the baby was found to be 0.02 m/s. It can be concluded that a uniform temperature distribution was obtained inside the incubator. According to the Figure 3(b), the highest velocity was obtained at the top of the incubator that is not affected to the baby.

Temperature and velocity distributions for the laminar flow type incubator were shown in Figures 4(a) and 4(b). Air is introduced into the flow field with a temperature of 34[degrees]C and the temperature on the baby was found to be 36[degrees]C. The boundary condition of the surface, representing the baby, was given as 36.5[degrees]C. It can be concluded that a heat loss was occurred on the baby. However, it can be adjusted by increasing the temperature of the air that is higher than 34[degrees]C. It is obvious that the air flow behaves like a cushion on the baby. The air velocity on the baby was found to be 0.1 m/s which is less than the velocity limit, indicated in the standard TS EN 60601-2-19. A particle study was also carried out in order to evaluate the effects of the particle penetrations. Particles were modeled as PM 10 physical parameters. The animation results show that none of the particles were able to penetrate inside the incubator.

According to the Figure 4(a), the air temperature on the baby was maintained above 34[degrees]C and uniform temperature distribution was obtained around the baby. The velocity was increased above the baby due to plate mounted at the upper part of the air inlet. This plate is used to increase the air velocity at the top of the flow field and functions as an air cushion generator above the baby.

The velocity results were obtained for different inlet air velocities. Figure 5 shows the velocity distribution for various heights inside the air flow field. The baby was located between 325-625 mm and the air flows in the direction of decreasing x values. According to the results, the velocity on the baby is around 0.05 m/s for all inlet air velocities. However, it can be concluded that the velocity should be between 0.05 m/s to 0.1 m/s at the air inlet.

EXPERIMENTAL STUDIES

In the experimental part of the study, closed type incubator was tested initially in order to examine the velocity profile inside the incubator. According to the standards, TS EN 60601-2-19 April 2012, it was given that the velocity above the baby should be less than 0.1 m/s. In the experiments incubator test unit was used.

In the first step of manufacturing of the laminar flow type incubator the plexiglas cover was removed. A HEPA cabinet was used in the manufacturing process that was mounted at one side of the incubator. Straws were located inside the HEPA cabinet in order to flatten the air flow. A photo during the experiments of the laminar flow type incubator is given in Figure 6(a). The air velocity was measured to be 0.4-0.5 m/s at the outlet of the HEPA cabinet. However, the air velocity was measured to be 0.06 m/s, as shown in Figure 6(b). Although the incubator is open to the environment, the air velocity was found to be less than the closed type incubator. It means that the laminar flow type incubators accomplish the negative effects of the closed type incubators.

CONCLUSIONS

In this study, a laminar flow type newborn incubator was investigated. In the numerical part of the study, initially a closed type incubator was modeled. The temperature and velocity distribution inside the incubator were obtained. In the second part of the numerical study, a laminar flow unit mounted incubator was modeled. The results show that a uniform air flow field can be obtained. A particle study was also carried out in order to determine the particle penetration inside the flow filed. Even though these devices are used in acclimatized environments, the particle study shows that none of the particles were able to penetrate into the flow field. In the experimental part of the study, the laminar flow type incubator was manufactured. A closed type incubator was used for the manufacturing. The plexiglas cover was removed and a HEPA cabinet was mounted. The air velocity results are in good agreement with numerical results which shows that these types of incubators can be an alternative for newborn neonatal.

ACKNOWLEGMENT

This study was funded by Turkish ISIB Turkish HVAC-R Exporters.

REFERENCES

Rojczyk M. and Szczygiel I. 2011. Numerical and experimental analysis of infant radiant warmer. Computer Methods in Mechanics, CMM 2011, Warsaw, Poland.

Central Intelligence Agency (CIA). 2015. https://www.cia.gov/library/publications/the-world-factbook/

Hippolite O. A. 2012. Neonatal Thermoneutrality in a Tropical Climate, Current Topics in Tropical Medicine, Dr. Alfonso RodriguezMorales(Ed.), ISBN:978-953-51-0274-8, InTech, Available from: http://www.intechopen.com/books/current-topicsin-tropical-medicine/neonatal-thermoneutrality

ASHRAE Fundamentals Handbook, Physiological principles, comfort and health, Atlanta 1989.

Wongkamhang A., Phasukkit P., Airphaiboon S., Pintavirooj C., Thongpance N., Sanpanich A., 3D Finite element analysis of heat transfer efficiency in a double wall infant incubator, The 2012 Biomedical Engineering International Conference.

Rojczyk M and Szczygiel I. 2013. Numerical analysis of radiant warmer, Computer Assisted Methods in Engineering and Science, 20: 237-265.

Fic MA, Ingham DB, Ginalski MK, Nowak AJ, Wrobel L. 2010. Heat and mass transfer under an infant radiant warmer-development of numerical model, Medical Engineering and Physics 32:497-504.

Ginalski MK, Nowak AJ, Wrobel LC. 2007. A combined study of heat and mass transfer in an infant incubator with an overhead screen, Medical Engineering and Physics 29:531-541.

Perez JMR, Golombek SG, Fajardo C, Sola A. 2013. A laminar flow unit for the care of critically ill newborn infants, Medical Devices: Evidence and Research 6:163-167.

TS EN 60601-2-19. Particular requirements for basic safety and essential performance of infant incubators. April 2012.

M. Zeki Yilmazoglu, PhD

Atilla Biyikoglu, PhD

Member ASHRAE, GGAC

Author Yilmazoglu is working as an energy director for Gazi University Hospital and researcher in Dept. of Mechanical Eng., Gazi University, Ankara, Turkey. Author Biyikoglu is a professor in the Department of Mechanical Engineering, Gazi University, Ankara, Turkey.

Caption: Figure 1 Infant mortality rate in 2013 (CIA 2015)

Caption: Figure 2 (a) Closed type incubator (b) Incubator with radiant heater

Caption: Figure 3 (a) Temperature and (b) Velocity distributions inside the closed type incubator

Caption: Figure 4 (a) Temperature and (b) Velocity distributions inside the laminar flow type incubator

Caption: Figure 5 Velocity results for different heights inside the laminar flow field

Caption: Figure 6 (a) Laminar flow type incubators and (b) Air velocities around the baby
Table 1. B Boundary conditions for a Closed Type Incubator

          Air Inlet     Air Inlet
         Temperature    Velocity
         ([degrees]C)     (m/s)

Case 1        32          0.41
Case 2        34          0.41
Case 3        36          0.41

Table 2. Boundary conditions for a laminar flow type incubator

          Air Inlet    Air Inlet
         Temperature   Velocity
         ([degrees]C)    (m/s)

Case 1       36          0.30
Case 2       34          0.30
Case 3       36          0.20
Case 4       36          0.10
Case 5       36          0.05
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Author:Yilmazoglu, M. Zeki; Biyikoglu, Atilla
Publication:ASHRAE Transactions
Date:Jan 1, 2017
Words:2445
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