The Original Model of Togari et al

In this model, a zone is divided into a finite number of horizontal layers. The horizontal temperature of each layer is assumed to be uniform, except for the regions affected by supply air jet ventilation. Each layer consists of a core cell and wall cells, as shown in Figure 5.10. The core cell represents the mainstream air flow. Where the layer is bounded by a wall, a virtual wall cell is defined which accounts for the mass flows in the boundary layer at the wall. The wall cell is called ‘virtual’ as no separate volume or cell is defined in the geometric model of the thermal zone.

There is an exchange of mass and heat Between a “wall cell” and a “core cell”. The method considers the three most significant paths for the heat and mass transfer (Figure 5.10): (1) heat and mass transfer between the layer and a wall surface, (2) heat transfer and air movement between the different layers and (3) heat transfer through the wall. Flows are defined as positive in the upward direction and from the wall cells to the layers.

Figure 5.10: Schematic representation of the division of the zone into core cells and wall cells and the three major paths for heat and mass transfer.

A flow chart of the calculation procedure is shown in Figure 5.11. Firstly, data about the geometric model and initial conditions are specified. These initial conditions are the layer temperature and mass flow between the layers. Secondly, at every time step changed boundary conditions and wall surface temperatures need to be provided. Once all these values are known, the thermal-zonal model can start the calculation by solving the different sub-models one by one.

Wall

cell Core cell Wall

cell

Core cell

Core cell (2) (1)

(3)

When a primary airstream is present (e.g. a jet) the mass flow and temperature of the primary airstream at the position entering a layer, first need to be calculated using layer temperatures of the preceding time step. One iteration loop then starts in which the temperature for every layer is calculated. The iteration loop consists of three sub-models. To start, the mass and heat transfer along interior surfaces of walls is calculated, which accounts for the near-wall flow. Next, the mass and heat balance for every core cell is determined by the sub-model for mass flow and by the sub-model for heat flow. Based on these three sub-models, the temperature for every layer is determined. This iteration loop is repeated until the algorithm converges to a solution.

Figure 5.11: Flow chart used by Togari et al.

Start

Data input + Initial Conditions Boundary Conditions at each time step Calculation of surrounding wall temperatures

Air conditioned

Primary airstream calculation using preceding time step layer temperatures

yes

Calculation of wall surfaces air currents Mass balance equation Heat balance equation

Calculate temperature layer Ti

Ti = Ti, previous

no

yes

End

no One time step

One iteration loop

MODELLING INDOOR CLIMATE IN TALL BUILDINGS 129

Sub-model for mass and heat transfer along interior surfaces of walls

If the inner surface temperature is not the same as the indoor temperature, a downward or upward airflow will be induced near the vertical wall. The heat transfer and mass flow along the surface are modelled using a so-called wall current model. This model assumes that the convective heat transfer drives a mass flow (mout(I,K)) with an average temperature (TD(i,K)) from layer i (core cell) to its related boundary layer at wall K (virtual wall cell).

To calculate the mass flow mout(i,K) and the temperature TD(i,K), the following assumptions are made:

 The mass transfer along the wall is modelled using a wall current model, based on the boundary layer theory for natural convection for a vertical wall. More specifically, it is based on the 1/7-law described by Eckert and Jackson [334] which counts for turbulent free convection along a flat plate. Based on their theory, the flow temperature of the air entrained from layer i to the corresponding current of wall K is calculated as follows (see appendix 5A):

TD(i,K)= 0.75T(i)+ 0.25Tw(i,K) (5.6)

 The rise or fall in temperature TD(i,K) in the boundary layer is dependent by the heat flow to the wall. To calculate the mass flow mout(i,K), the equation for a convective heat flow (q_w(i,K)) between a surface and its environment is used (5.7). By setting this heat flow equal to the heat flow leaving or going into layer i (5.8), and replacing TD(i,K) by the equation from above (eq. 5.6), mout(i,K) can be calculated as following:

q_w(i,K)= h_C(i,K). A(i,K). (T_w(i,K)− T(i)) (5.7) q_w(i,K)= C. m_out(I,K). (T_D(i,K)− T(i)) (5.8)

m_out(i,K)= 4 h_C(i,K). A(i,K)

C (5.9)

Where q_w(i,K) heat flow [W]

Tw(i,K) average surface temperature of the wall K adjacent to layer i [ °C]

Ti average temperature of the layer i [°C]

T_D(i,K) bulk temperature of the mass flow m_out(I,K) [°C]

h_C(i,K) heat convection coefficient [W/m²K]

Aw(i,K) area of wall K adjacent to layer i [m²]

C_i specific heat of the air in layer i [J/kg.K]

mout(I,K) mass flow leaving layer I to wall K [kg/s]

Figure 5.12: Schematic representation of the wall currents and air mass flows in one layer in case there is a downward stream along the wall. Flow streams belonging to layer I and wall K are indicated in blue.

If the current is downward, as shown in Figure 5.12, the current flows from wall cellK,i+1 to wall cellK,i. This flow will be mixed with the flow mout(I,K) coming from layer i. The same analogy holds if the current is upwards. The mass mm(i,K) and an average temperature Tm(i,K) of the mixed flow is calculated as following:

mm(i,K)= mout(i,K)− min (mmd(i+1,K), 0) + max(mmd(i−1,K), 0) (5.10) T_m(i,K)

=m_out(i,K)T_D(i,K)− min(m_md(i+1,K), 0) T_m(i+1,K)+ max (m_md(i−1,K), 0)T_m(i−1,K)

m_m(i,K) (5.11)

Where mm(i,K) mass of the wall current of wall K adjacent to layer i [kg/s]

m_md(i±1,K) mass of the wall current of wall K coming from the layer above (i+1) or the layer below (i-1). The value is negative in case this current is downward and positive in case this current is upward [kg/s]

T_m(i,K) the temperature of the wall current of wall K adjacent to layer i [°C]

Some of the air of this current mm(i,K) may return to the air layer i (min(i,K)) and some may continue to the cell above or below (mmd(i,K)), depending on the direction of the current flow.

The splitting of the mass m_m(i,K) into m_in(i,K) and m_md(i,K) is defined by the ratio P(i,K).

TD(i,K)

m_md(i,K)

T_m(i,K) 𝑇_(𝑖−1)

mmd(i+1,K)

T_m(i+1,K) 𝑇_(𝑖+1)

T_(i) m_m(i,K)

T_m(i,K)

m_in(i,K) T_m(i,K) m_out(i,K)

Tw(i,k)

MODELLING INDOOR CLIMATE IN TALL BUILDINGS 131

m_in(i,K)= (1 − P_i,K)m_m(i,K) (5.12)

m_md(i,K)= P_i,Km_m(i,K) (5.13)

In case the airstream is downward, the ratio P(i,K) is dependent on the relationship between the air temperature in layer i and in layer i-1. When the airstream is upwards, the ratio is dependent of the temperature in layer i and layer i+1. Table 5.3 presents the criteria for the airflow pattern, and how the value for ratio P(i,K) is determined. The justification is described or a downward airstream along the wall. The same justification holds in case the airstream is upwards. When the temperature of the mixed air current T_m(i,K)is higher than the temperature in the same layer, the mixed airstream will flow back into the core cell. When the temperature of the mixed air current T_m(i,K) is lower than temperature in the same layer, but higher than the temperature of the next layer, a part of the mixed stream will flow back into the core cell, and a part will flow downwards along the wall. In case the temperature of the mixed air current T_m(i,K)is lower than the temperature of the same layer and the temperature of the next layer, the mixed stream will completely flow into the next layer, where it will entrain mass mout(I,K) of the layer i [335].

Table 5.3: Criteria for the airflow pattern.

Flow direction Temperature conditions Pi,K

Descending T_m(i,K)≥ T(i) 0

Sub-model for mass balance between horizontal layers

Figure 5.13 represents the different mass flows from and to core cell of the layer. The calculation of the mass balance in every layer begins with the lowermost layer. The mass balance for layer i is calculated by:

m_layer net air flow to the layer underneath (m_i−1) and above( m_i) kg/s

=mi−1− mi [kg/s]

mout(i,K) mass flow from layer i to wall current of wall K, kg/s min(i,K) mass flow from wall current of wall K to layer i, kg/s

Figure 5.13: Schematic of the wall currents and air mass flows in one layer in case there is a downward stream along the wall.

Sub-model for heat balance between horizontal layers Similar to the mass flows, the heat balance can be written as:

Vi

(ρC)T_i− (ρC)T_i^prev

∆t = Qsource+ Qlayer+ Qcurrents+ Qb (5.15)

Where V_i volume of layer i [m³]

∆t time step [s]

prev value of the previous time step Q_source heat sources or sinks [W]

Q_layer heat flow due to mass transport between layer i and layer i-1 and between layer i and layer i+1 [W]

Qcurrent heat flow between the core cell and the wall currents representing the heat flow by convection along vertical walls [W]

Qb heat flow by temperature difference between adjacent layers and by penetrative convection between adjacent layers. In case of inversion between layers, the heat flow becomes larger due to instability

TD(i,K)

T(i−1)

T(i+1)

T(i)

m_in(i,K) T_m(i,K) mout(i,K)

m(i−1)

m_(i)

MODELLING INDOOR CLIMATE IN TALL BUILDINGS 133

As can be seen in the heat balance equation a term Qb is added. This term represents the heat flow by by penetrative convection between two adjacent layers. In case of inversion between layers, the heat flow becomes larger due to instability. The term is calculated as follows:

Qb= CbAb(T(i−1)− T(i)) (5.16)

Where: C_b the convective heat transfer coefficient at the interface between two layers [319]

A_b cross section area of the top or bottom layer [m²]

Writing out the individual terms in the equation, the heat conservation equation becomes:

V_i(ρC)T_i− (ρC)Tiprev derived from the former described equations. It must be mentioned that for the bottom and top layer, convective heat transfer along the floor and ceiling surfaces are also included. The value for the surface Ab is obtained from the geometric model. The only unknown is the value for the heat transfer factor Cb.

Togari et al. [66] suggested the following values for the term Cb: when a stable temperature stratification is formed, 2.3W/m²K, and in case an unstable temperature stratification is formed, 112W/m²K. However, Togari et al. [66] stated that further research was needed to determine a more accurate value for Cb. From that point of view, these authors started a sensitivity analysis.

They observed that the value of Cb had a small effect on the vertical temperature difference as long as air transfer is active between the layers. However, in case stable temperature stratification is formed and air flow rates between layers become small, the value of Cb has a larger effect on the vertical temperature difference. In this context, Gao et al. [319] searched for a physical definition of the heat transfer factor. They stated that the one-dimensional heat transfer Qb represents the heat transfer between adjacent air layers due to the combined diffusion of laminar and turbulent conduction and that the term Cb can be expressed by the following theoretical equation:

C_b= μ_m(λa

lμ_m+ c_p. μ_t

Pr. l. μ_m) (5.18)

Where: λ_a thermal conductivity of air [W/(m.K)]

l a characteristic length, defined as the height of a layer [m]

Pr Prandtl number [-]

μ_mand μt molecular and turbulent viscosity

Based on this formula, they found a linear relation between the factor Cb and the turbulent viscosity ratio μ_t/μ_m, shown in Figure 5.14. The value for Cb is calculated by the CFD method.

Figure 5.14: Linear relationship between the heat transfer factor Cb and turbulent viscosity ratio 𝜇_𝑡/𝜇_𝑚, provided by Gao et al. [319].

In document Preventive conservation of works of art in historic buildings : assessment and modelling (Page 155-162)