Non-homogeneous Thermal Properties of Bamboo

Non-homogeneous Thermal Properties of Bamboo Puxi Huang1, Wen-Shao Chang1, Andy Shea1, Martin P. Ansell2, and Mike Lawrence1 1

BRE CICM, Dept. Architecture & Civil Eng., University of Bath, UK {P.Huang,W.chang3,A.shea,M.lawrence}@bath.ac.uk 2 BRE CICM, Dept. Mechanical Eng., University of Bath, UK [email protected]

Abstract. A Phyllostachys edulis (Moso Bamboo)sample’s density, heat capacity and thermal effusivity were obtained by a series of experiments. The porosity, thermal conductivity and thermal diffusivity were calculated. Based on these experimental values, this study discusses the Phyllostachys edulis sample’s microstructure characteristics and the causes of the variation of thermal properties along the radial direction. Keywords: Moso bamboo, heat capacity, thermal effusivity, conductivity and diffusivity, porosity, variation along radial direction, heat transfer models.

1

Introduction

Bamboo has been widely acknowledged and appreciated as an environmental building material for its growing speed, carbon sequestration and mechanical properties (Flander 2009). In terms of energy saving, a building material’s thermal performance parameters are often decisive. However, the following reasons mean that bamboo’s thermal performance data is difficult to obtain: • Bamboo culm is a hollow cylinder rather than a solid. Therefore, flat board based thermal performance test equipment cannot be utilised to test the curved surface. • Although raw bamboo can be manufactured into flat samples, the size is still restricted by its thickness and radius. Accuracy could not be assured in a number of main current stationary thermal parameters tests. In addition, the curved bamboo cutoff material’s thermal performance contribution is sacrificed in the procedure of ‘the rectangle manufacture’. • Bamboo is a heterogeneous material and the changes in density and porosity can cause thermal property variations.

S. Aicher et al. (eds.), Materials and Joints in Timber Structures, RILEM Bookseries 9, DOI: 10.1007/978-94-007-7811-5_59, © RILEM 2014

657

658

P. Huang et al.

Field study indicates that this variation is ignored in the rough manufacture process. Outer green and inner yellow skin are planed along the radial direction without considering adjusting for the thermal performance. In order to discuss the Phyllostachys edulis made window-frame, prefabricated wall panel and floor slab’s thermal performance, a series of research works are predicated. This study aims to provide the crucial simulation parameters to the heat transfer modeling work.

2

Anatomical Observation of Phyllostachys Edulis in Radial Direction

Anatomically, Phyllostachys edulis culms in radial direction can be classified into four zones. The first zone is the Peripheral zone, namely the epidermis layer consisting of the hypodermis and primary cortex tissue. Two zones form the middle part, the Transitional zone and the Central zone. Both of these are composed of vascular bundles and parenchymatous ground tissue. The difference is that the vascular bundles remain undifferentiated in the Transitional zone, while they appear fully differentiated in the central zone. The fourth zone is the Inner zone. The character of this zone is that simplified vascular bundles start to appear nondirectionally (Grosser and Liese 1971, Liu ed al. 2012). Microscope observation indicates that the density of the Phyllostachys edulis culms is inconsistent in radial direction. The Peripheral zone’s hypodermis and primary cortex tissue is distinct from the other zones. The vascular bundles’ morphological variation and the proportion of parenchymatous ground tissue dominate density change in the other three zones. The metaxylem vessel cavity of the vascular bundles is the main channel of water transportation. The vascular bundles’ metaxylem vessel cavity area is measured by Image J. The boundary needs to be demarcated manually, and then the area and centre point coordinates are calculated automatically. The figure 1 could illustrate every metaxylem vessel cavity’s position distribution and area change. This prior study utilises the metaxylem vessel area so as to calculate porosity. The overall porosity of this sample is 4.06%. The figure 1 indicates that more small vascular bundles’ metaxylem vessels appear in the transitional zone. From central zone to inner zone, the individual metaxylem vessel distributes more sparsely whilst the vessel’s area increases gradually. The air cavity’s position and area distribution could change the thermal conductivity. For instance, if a large cavity area appears at a high density area, this area’s density will be counteracted; the thermal conductivity will decrease accordingly. The density’s variation characteristics will be further discussed in the next section.

Non-homogeneous Thermal Properties of Bamboo

659

Fig. 1 The metaxylem vessel cavity’s position distribution and density variation

3

Heat Transfer Parameters

Heat transfer may be described by the following Partial Differential Equation (PDE). ρC

T

=k

T

(1)

ρ: Density (kg/m3) C : Specific heat capacity (J/kg K) k: Thermal Conductivity (W/m K) T: Temperature (K) t: Time (S) The thermal conductivity, specific heat capacity and density of the Phyllostachys edulis sample need to be obtained in order to satisfy the PDE equation’s definite condition (Incropera 1981).

3.1

Density

In order to quantify the density variation tendency, a CT scanner is utilised to capture a grey scale image of the Phyllostachys edulis samples. Benchmark grey scale value is provided by Balsa, Polypropylene, water and Aluminum. The CT scan direction is from the Phyllostachys edulis sample’s internal surface to its external surface. The sample’s thickness is 10.084mm. Therefore, the sample’s absolute radial density value can be positioned along this direction. As demonstrated by figure 1, high density values appear from the external side to the

660

P. Huang et al.

subsequent 2.5mm zone. The reason for this is that the zone constitutes hypodermis and primary cortex tissue and non-differential vascular bundles. Then the density values fluctuate and decrease along the radial direction. The peak density values are attributed by the vascular bundles’ sclerenchyma sheaths and parenchymatous ground tissue’s cell wall. The vascular bundles’ cavity is the result of bottom values. Due to parenchyma cells’ aggregation, the density value rises again at the internal surface side.

3.2

Heat Capacity

The heat capacity of three Phyllostachys edulis samples was tested by the Differential Scanning Calorimeter (DSC). The samples’ dimensions and cutting positions are illustrated by figure 2. The external sample and internal sample are exactly cut at external surface and internal surface respectively. The middle sample’s center point coincides with the whole sample’s middle point at the radial section. A specific manufacture device was developed to control the thickness of the sample while keeping a smooth surface. The manufactured samples could well fill into the DSC test pan.

Fig. 2 DSC test samples

Fig. 3 Heat capacity of the test samples


661

All of the samples are pre-heated to eliminate water. At the same temperature, the middle sample has the highest heat capacity value, followed by the internal sample. The external sample’s heat capacity is the lowest. Every sample’s heat capacity curve has its peak value. The internal, middle and external sample’s heat capacity peak values appear at 70.35℃, 64.68℃ and 80.32℃ respectively. The DSC heat capacity test result indicates that the heat capacity is variable with the temperature. Due to relatively low density and high porosity in the middle zone, the middle sample’s heat capacity value is higher than the internal sample and external sample. Every sample’s heat capacity value increases with temperature until achieving the peak value.

3.3

Thermal Effusivity

The thermal effusivity of six Phyllostachys edulis samples is tested by the CTherm (sensor T301) TCi system. This system is based on the Modified Transient Plane Source (MTPS) method (Parlouer 2013). The samples are cut from the same Phyllostachys edulis culm along the radial section. Each sample’s dimension and cutting position are shown by figure 4. The sample’s thickness is 5mm. Both length and width are 25mm. The external sample’s upper surface is the outer green of the Phyllostachys edulis. The internal sample’s bottom surface is the inner skin. The other samples are cut stepwise between the external sample and internal sample. Each step height is 1mm.

Fig. 4 Thermal effusivity test result

662

P. Huang et al.

Sample 3’s effusivity value reveals an obvious distinction among the six samples. The other 5 samples have a similar tendency in that the effusivity value decreases along the radial direction from external surface to internal surface. The reason for this is that a sample’s density starts to decrease while the vascular bundles cavity’s number still remains at a relatively high level at around 3mm. By these average test values, the thermal effusivity along the radial direction can be calculated via a data fitting process. The thermal effusivity variation tendency is shown by figure 5.

Fig. 5 The sample’s thermal effusivity variation from external surface to internal surface

High values appear at 1.5mm from the external surface, then effusivity rapidly decreases to the bottom value. From the 2.5mm to 5.5mm zone, the effusivity slightly rebounds to 440 Ws0.5/m2K. The vascular bundle’s number declines in this zone. The second descent zone ranges from 5.5mm to 8.5mm. The effusivity value has a rising trend at the last 1.5mm. The fluctuation interrelates closely with the density and porosity variation. The effusivity value will be utilised to calculate the thermal conductivity value with heat capacity and density value in the simulation process.

4

Thermal Conductivity and Thermal Diffusivity

4.1

Thermal Conductivity Calculation

Since the samples thermal effusivity, density and heat capacity can be set as variables of radial length, the formula 2 may be utilised to calculate the thermal conductivity. k = e /ρC (2) e: Thermal effusivity (Ws0.5/m2K) The external surface’s and internal surface’s thermal conductivity are remarkably higher than that of the middle 8mm zone and this is due to high thermal effusivity and low heat capacity. The average thermal conductivity of this sample is 0.227W/m K. If using average thermal effusivity, density and heat capacity value to calculate average thermal conductivity, the result is 0.162 W/m K. In which case, the thermal conductivity is underestimated.


663

Fig. 6 The sample’s thermal conductivity variation from external surface to internal surface

4.2

Thermal Diffusivity Calculation

Thermal diffusivity can be utilised to quantify temperature balance speed. The formula 3 describes the relationship between thermal diffusivity, thermal conductivity, density and heat capacity. α = k/ρC

(3)

2

α: Thermal diffusivity (m /s)

Fig. 7 The sample’s thermal diffusivity variation from external surface to internal surface

If the tested variable values are utilised to calculate thermal diffusivity, the result is also a variable series. The average value of the series is 6.18×10-7 m2/s. If average values of thermal conductivity, density and heat capacity are utilised to calculate thermal diffusivity, the result is 2.37×10-7 m2/s. Therefore, simply using the average value of a whole Phyllostachys edulis sample is inaccurate to reflect the heat transfer process along the radial direction.

5

Summary and Future Works

We can see that the Phyllostachys edulis sample’s microstructure is variable along the radial direction from anatomical observation. The porosity is dominated by the

664

P. Huang et al.

metaxylem vessel’s area and geometry distribution. The thermal conductivity and thermal diffusivity calculation results indicate that it is not appropriate to describe Phyllostachys edulis sample’s heat transfer process using average thermal parameters. In terms of the heat transfer through heterogeneous material, like bamboo, the density, heat capacity, and thermal effusivity vary and these variations cause thermal conductivity and thermal diffusivity fluctuation along the radial direction and are crucial factors in the heat transfer procedure. The subsequent work will be further developed in the following fields: Numerical heat transfer models from radial, longitudinal and tangential directions will be built to simulate the heat transfer through these directions. The Phyllostachys edulis sample from different positions will be tested to build a thermal parameters database. The proving tests and database will be continuously utilised so as to modify the numerical model’s error.

References Flander, K.D., Rovers, R.: One laminated bamboo-frame house per hectare per year. Construction and Building Materials 23(1), 210–218 (2009), doi: http://dx.doi.org/10.1016/j.conbuildmat.2008.01.004 Grosser, D., Liese, W.: On the anatomy of Asian bamboos, with special reference to their vascular bundles. Wood Science and Technology 5(4), 290–312 (1971), doi: 10.1007/ bf00365061 Incropera, F.P.: Fundamentals of heat transfer. Wiley, New York (1981) Liu, K., Takagi, H., Osugi, R., Yang, Z.: Effect of physicochemical structure of natural fiber on transverse thermal conductivity of unidirectional abaca/bamboo fiber composites. Composites Part A: Applied Science and Manufacturing 43(8), 1234–1241 (2012), http://dx.doi.org/10.1016/j.compositesa.2012.02.020 Parlouer, P.L.: Thermal Conductivity of six bamboo sections SETARAM Instrumentation, Caluire, H5793 (2013)